Change running subscriptions

AppSubscription::change() changes price, unit, name, description or
discount of a running subscription, prorated right away (APPLY_NOW) or
from the next period on (APPLY_PERIOD_END, shown as scheduled_change);
cancelScheduledChange() drops a scheduled one. Adds the
subscription.updated event type and documents proration_not_possible
and credit_exceeds_bill.

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);

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());
     }
 }