diff --git a/README.md b/README.md
index 21e45b4..cc8f690 100644
--- a/README.md
+++ b/README.md
@@ -562,7 +562,8 @@ subscriptions it created (plus those of clients delegated to it).
 Failures throw `Anikeen\Id\Exceptions\ApiException` with `status` (0 = not reachable), a stable
 `error` code (`already_active`, `already_cancelled`, `not_active`, `already_paused`,
 `already_resumed`, `billing_address_missing`, `not_changeable`, `subscription_not_found`,
-`idempotency_key_conflict`, `in_checkout_session`, `trial_already_used`) and `isNotFound()`, `isConflict()`, `isTemporary()`:
+`idempotency_key_conflict`, `in_checkout_session`, `trial_already_used`, `proration_not_possible`,
+`credit_exceeds_bill`) and `isNotFound()`, `isConflict()`, `isTemporary()`:
 
 ```php
 try {
@@ -578,6 +579,43 @@ try {
 }
 ```
 
+### Changing running subscriptions (trusted apps)
+
+`update()` changes a subscription before its checkout (afterwards only the description).
+`change()` changes price, unit, name, description or discount of a running subscription, like a
+plan change (send only what changes):
+
+```php
+use Anikeen\Id\Resources\AppSubscription;
+
+// upgrade now, prorated: the unused rest of the period is credited, the new price charged for it
+$subscription->change(['apply' => AppSubscription::APPLY_NOW, 'price' => '0.0274', 'name' => 'Cloud Server M']);
+
+// downgrade from the next period on
+$subscription->change(['apply' => AppSubscription::APPLY_PERIOD_END, 'price' => '4.99']);
+$subscription->scheduled_change;       // {applies_at, name, description, unit, price, discount, created_at}
+$subscription->cancelScheduledChange(); // keep the current values after all
+```
+
+- `now` credits the unused part of the current period at the rate it was billed at and charges
+  the rest at the new price, both on the user's open bill. A new `unit` starts a new, fully billed
+  period now. A downgrade whose credit would exceed the open bill is refused with 409
+  `credit_exceeds_bill`: schedule it for the period end instead. 409 `proration_not_possible` when
+  the current period wasn't billed to the subscription (e.g. a takeover); `period_end` works.
+- `period_end` changes nothing now; the next period is billed at the new values. A new
+  `period_end` change replaces the scheduled one, `now` drops it.
+- During a free trial nothing was billed: both just change what the trial ends in.
+- `discount` replaces the discount from the next billed period on, `null` ends it. When the unit
+  changes, a discount that counts in periods or an `amount_off` has to be sent again (422).
+- Subscriptions with a monthly cap change only with `period_end`.
+- Only running subscriptions can change: 409 `not_changeable` for drafts (use `update()`), paused,
+  cancelling, expired and cancelled ones.
+
+When a change takes effect (right away or at the period end), Anikeen ID sends
+`subscription.updated` (`EventType::SUBSCRIPTION_UPDATED`) with the old values of the changed
+fields in `$event['data']['previous_attributes']`. Telling the user about a price increase (and
+getting consent where needed) is up to your app.
+
 ### App orders (trusted apps, no user token)
 
 Orders that bill the app's subscriptions, read with the app's token (scope `orders:client`):
diff --git a/README.stub b/README.stub
index fc74eb8..df8e484 100644
--- a/README.stub
+++ b/README.stub
@@ -562,7 +562,8 @@ subscriptions it created (plus those of clients delegated to it).
 Failures throw `Anikeen\Id\Exceptions\ApiException` with `status` (0 = not reachable), a stable
 `error` code (`already_active`, `already_cancelled`, `not_active`, `already_paused`,
 `already_resumed`, `billing_address_missing`, `not_changeable`, `subscription_not_found`,
-`idempotency_key_conflict`, `in_checkout_session`, `trial_already_used`) and `isNotFound()`, `isConflict()`, `isTemporary()`:
+`idempotency_key_conflict`, `in_checkout_session`, `trial_already_used`, `proration_not_possible`,
+`credit_exceeds_bill`) and `isNotFound()`, `isConflict()`, `isTemporary()`:
 
 ```php
 try {
@@ -578,6 +579,43 @@ try {
 }
 ```
 
+### Changing running subscriptions (trusted apps)
+
+`update()` changes a subscription before its checkout (afterwards only the description).
+`change()` changes price, unit, name, description or discount of a running subscription, like a
+plan change (send only what changes):
+
+```php
+use Anikeen\Id\Resources\AppSubscription;
+
+// upgrade now, prorated: the unused rest of the period is credited, the new price charged for it
+$subscription->change(['apply' => AppSubscription::APPLY_NOW, 'price' => '0.0274', 'name' => 'Cloud Server M']);
+
+// downgrade from the next period on
+$subscription->change(['apply' => AppSubscription::APPLY_PERIOD_END, 'price' => '4.99']);
+$subscription->scheduled_change;       // {applies_at, name, description, unit, price, discount, created_at}
+$subscription->cancelScheduledChange(); // keep the current values after all
+```
+
+- `now` credits the unused part of the current period at the rate it was billed at and charges
+  the rest at the new price, both on the user's open bill. A new `unit` starts a new, fully billed
+  period now. A downgrade whose credit would exceed the open bill is refused with 409
+  `credit_exceeds_bill`: schedule it for the period end instead. 409 `proration_not_possible` when
+  the current period wasn't billed to the subscription (e.g. a takeover); `period_end` works.
+- `period_end` changes nothing now; the next period is billed at the new values. A new
+  `period_end` change replaces the scheduled one, `now` drops it.
+- During a free trial nothing was billed: both just change what the trial ends in.
+- `discount` replaces the discount from the next billed period on, `null` ends it. When the unit
+  changes, a discount that counts in periods or an `amount_off` has to be sent again (422).
+- Subscriptions with a monthly cap change only with `period_end`.
+- Only running subscriptions can change: 409 `not_changeable` for drafts (use `update()`), paused,
+  cancelling, expired and cancelled ones.
+
+When a change takes effect (right away or at the period end), Anikeen ID sends
+`subscription.updated` (`EventType::SUBSCRIPTION_UPDATED`) with the old values of the changed
+fields in `$event['data']['previous_attributes']`. Telling the user about a price increase (and
+getting consent where needed) is up to your app.
+
 ### App orders (trusted apps, no user token)
 
 Orders that bill the app's subscriptions, read with the app's token (scope `orders:client`):
diff --git a/src/Id/Resources/AppSubscription.php b/src/Id/Resources/AppSubscription.php
index 5ea2150..3c7f622 100644
--- a/src/Id/Resources/AppSubscription.php
+++ b/src/Id/Resources/AppSubscription.php
@@ -37,11 +37,18 @@ use JsonSerializable;
  * @property ?array $payload
  * @property ?string $idempotency_key
  * @property ?string $checkout_session_id the checkout session the user confirms it in, if any
+ * @property ?object $scheduled_change {applies_at, name, description, unit, price, discount, created_at}: a change() for the period end
  */
 class AppSubscription implements JsonSerializable
 {
     use HasSubscriptionStatus;
 
+    /** change(): prorated, right away. */
+    public const APPLY_NOW = 'now';
+
+    /** change(): from the next period on (scheduled_change until then). */
+    public const APPLY_PERIOD_END = 'period_end';
+
     public function __construct(protected object $data)
     {
     }
@@ -60,6 +67,7 @@ class AppSubscription implements JsonSerializable
      * Change a subscription (send only what changes). Before checkout everything can change;
      * afterwards only the description, which names what the customer pays for and is shown
      * on their bills, e.g. `$subscription->update(['description' => 'web-01.example.com'])`.
+     * Prices of running subscriptions change with change().
      *
      * @throws ApiException
      */
@@ -68,6 +76,51 @@ class AppSubscription implements JsonSerializable
         return $this->send('PUT', '', $attributes);
     }
 
+    /**
+     * Change a running subscription (send only what changes), like a plan change:
+     *
+     * - `apply: now`: prorated – the unused rest of the current period is credited at the rate it
+     *   was billed at and charged at the new price on the user's open bill. A new unit starts a
+     *   new, fully billed period now. A downgrade whose credit exceeds the open bill is refused
+     *   (409 "credit_exceeds_bill"): schedule it for the period end instead.
+     * - `apply: period_end`: nothing changes now; the subscription has `scheduled_change` and the
+     *   next period is billed at the new values. A new scheduled change replaces the previous one.
+     *
+     * During a free trial nothing was billed, so both just change what the trial ends in.
+     * Webhook `subscription.updated` (with `data.previous_attributes`) when the change takes effect.
+     *
+     * @param array{
+     *     apply: string,
+     *     price?: float|string,
+     *     unit?: string,
+     *     name?: string,
+     *     description?: null|string,
+     *     discount?: null|array{label: string, percent_off?: float|string, amount_off?: float|string, duration: string, duration_in_periods?: int}
+     * } $attributes apply: self::APPLY_NOW or self::APPLY_PERIOD_END.
+     *               unit: a recurring unit (hour, day, week, month, year).
+     *               discount: replaces the discount from the next billed period on, null ends it;
+     *               required with a new unit when the current one counts in periods or is an
+     *               amount_off. Subscriptions with a monthly cap change only at the period end.
+     *
+     * @throws ApiException 409 "not_changeable" (not running: draft, paused, cancelling, …),
+     *                      "proration_not_possible", "credit_exceeds_bill",
+     *                      "billing_address_missing"; 422 validation
+     */
+    public function change(array $attributes): self
+    {
+        return $this->send('PUT', '/change', $attributes);
+    }
+
+    /**
+     * Drop the change scheduled for the period end (nothing happens without one).
+     *
+     * @throws ApiException
+     */
+    public function cancelScheduledChange(): self
+    {
+        return $this->send('DELETE', '/change');
+    }
+
     /**
      * Activate the subscription and bill its first period.
      *
diff --git a/src/Id/Webhooks/EventType.php b/src/Id/Webhooks/EventType.php
index 77208f2..d394cb9 100644
--- a/src/Id/Webhooks/EventType.php
+++ b/src/Id/Webhooks/EventType.php
@@ -27,6 +27,12 @@ final class EventType
     /** The last discounted period of a subscription was billed; the following ones are billed at the full price. */
     public const SUBSCRIPTION_DISCOUNT_ENDED = 'subscription.discount_ended';
 
+    /**
+     * The price, unit, name, description or discount of a running subscription changed (right away
+     * or at the period end); `data.previous_attributes` has the changed fields' old values.
+     */
+    public const SUBSCRIPTION_UPDATED = 'subscription.updated';
+
     /** The user confirmed a checkout session: all of its subscriptions are running (sent after their subscription.activated). */
     public const CHECKOUT_COMPLETED = 'checkout.completed';
 
diff --git a/tests/Id/AppSubscriptionsTest.php b/tests/Id/AppSubscriptionsTest.php
index fbf1111..c9717e4 100644
--- a/tests/Id/AppSubscriptionsTest.php
+++ b/tests/Id/AppSubscriptionsTest.php
@@ -5,6 +5,7 @@ namespace Anikeen\Id\Tests;
 use Anikeen\Id\AnikeenId;
 use Anikeen\Id\Billable;
 use Anikeen\Id\Exceptions\ApiException;
+use Anikeen\Id\Resources\AppSubscription;
 use GuzzleHttp\Exception\ConnectException;
 use GuzzleHttp\Psr7\Request;
 use Illuminate\Database\Eloquent\Model;
@@ -215,6 +216,43 @@ class AppSubscriptionsTest extends TestCase
         $this->assertSame(3, $subscription->discount->periods_remaining);
     }
 
+    public function test_a_running_subscription_is_changed_now_or_at_the_period_end(): void
+    {
+        $change = ['applies_at' => '2026-12-01T00:00:00+00:00', 'name' => 'Pro', 'description' => null, 'unit' => 'month', 'price' => 14.99, 'discount' => null, 'created_at' => '2026-11-04T10:00:00+00:00'];
+        $this->respond(200, $this->subscription(['active' => true, 'price' => 14.99]));
+        $this->respond(200, $this->subscription(['active' => true, 'price' => 14.99, 'scheduled_change' => $change]));
+        $this->respond(200, $this->subscription(['active' => true, 'price' => 14.99, 'scheduled_change' => null]));
+
+        $subscription = new AppSubscription((object) $this->subscription(['active' => true])['data']);
+
+        $this->assertSame(14.99, $subscription->change(['apply' => AppSubscription::APPLY_NOW, 'price' => '14.99', 'discount' => null])->price);
+        $this->assertSame('2026-12-01T00:00:00+00:00', $subscription->change(['apply' => AppSubscription::APPLY_PERIOD_END, 'unit' => 'month', 'name' => 'Pro'])->scheduled_change->applies_at);
+        $this->assertNull($subscription->cancelScheduledChange()->scheduled_change);
+
+        $this->assertSame([
+            'PUT /api/v1/client/subscriptions/01JSUB/change Bearer app-token',
+            'PUT /api/v1/client/subscriptions/01JSUB/change Bearer app-token',
+            'DELETE /api/v1/client/subscriptions/01JSUB/change Bearer app-token',
+        ], array_map(fn ($entry) => $entry['request']->getMethod() . ' ' . $entry['request']->getUri()->getPath() . ' ' . $entry['request']->getHeaderLine('Authorization'), $this->sent));
+        $this->assertSame(['apply' => 'now', 'price' => '14.99', 'discount' => null], $this->sentJson(0));
+        $this->assertSame(['apply' => 'period_end', 'unit' => 'month', 'name' => 'Pro'], $this->sentJson(1));
+    }
+
+    public function test_a_refused_change_carries_its_error_code(): void
+    {
+        $this->respond(409, ['message' => 'The credit would exceed the open bill.', 'error' => 'credit_exceeds_bill']);
+
+        $subscription = new AppSubscription((object) $this->subscription(['active' => true])['data']);
+
+        try {
+            $subscription->change(['apply' => AppSubscription::APPLY_NOW, 'price' => '1.99']);
+            $this->fail('Expected an ApiException.');
+        } catch (ApiException $e) {
+            $this->assertTrue($e->isConflict());
+            $this->assertSame('credit_exceeds_bill', $e->error);
+        }
+    }
+
     public function test_a_missing_user_id_is_reported(): void
     {
         $this->expectException(InvalidArgumentException::class);
diff --git a/tests/Id/CheckoutSessionsTest.php b/tests/Id/CheckoutSessionsTest.php
index 764c3a5..675adad 100644
--- a/tests/Id/CheckoutSessionsTest.php
+++ b/tests/Id/CheckoutSessionsTest.php
@@ -187,5 +187,6 @@ class CheckoutSessionsTest extends TestCase
         $this->assertContains('subscription.activated', EventType::all());
         $this->assertContains('subscription.trial_will_end', EventType::all());
         $this->assertContains('subscription.discount_ended', EventType::all());
+        $this->assertContains('subscription.updated', EventType::all());
     }
 }