diff -u b/core/modules/system/tests/modules/new_dependency_test/new_dependency_test.install b/core/modules/system/tests/modules/new_dependency_test/new_dependency_test.install
--- b/core/modules/system/tests/modules/new_dependency_test/new_dependency_test.install
+++ b/core/modules/system/tests/modules/new_dependency_test/new_dependency_test.install
@@ -14,2 +14,7 @@
   \Drupal::getContainer()->get('module_installer')->install(['new_dependency_test_with_service']);
+
+  \Drupal::state()->set(
+    'new_dependency_test_update_8001.decorated_service',
+    \Drupal::service('new_dependency_test.another_service')->greet()
+  );
 }
diff -u b/core/modules/system/tests/modules/new_dependency_test/new_dependency_test.services.yml b/core/modules/system/tests/modules/new_dependency_test/new_dependency_test.services.yml
--- b/core/modules/system/tests/modules/new_dependency_test/new_dependency_test.services.yml
+++ b/core/modules/system/tests/modules/new_dependency_test/new_dependency_test.services.yml
@@ -12,0 +13,6 @@
+  new_dependency_test.another_service:
+    class: Drupal\new_dependency_test\Service
+  new_dependency_test.another_service.decorated:
+    class: Drupal\new_dependency_test\Service
+    decorates: new_dependency_test.another_service
+    arguments: ['@new_dependency_test.another_service.decorated.inner']
diff -u b/core/modules/system/tests/src/Functional/Update/UpdatePathNewDependencyTest.php b/core/modules/system/tests/src/Functional/Update/UpdatePathNewDependencyTest.php
--- b/core/modules/system/tests/src/Functional/Update/UpdatePathNewDependencyTest.php
+++ b/core/modules/system/tests/src/Functional/Update/UpdatePathNewDependencyTest.php
@@ -60,6 +60,8 @@
     $this->assertEquals('Hello World', $this->container->get('new_dependency_test.decorated')->greet());
     $this->assertEquals('Hello World', $this->container->get('new_dependency_test.decorated_optional')->greet());
 
+    // Tests that an existing decorated service works as expected during update.
+    $this->assertEquals('I am a decorator', \Drupal::state()->get('new_dependency_test_update_8001.decorated_service'));
   }
 
 }
only in patch2:
unchanged:
--- /dev/null
+++ b/core/modules/system/tests/modules/new_dependency_test/src/Service.php
@@ -0,0 +1,18 @@
+<?php
+
+namespace Drupal\new_dependency_test;
+
+class Service {
+
+  public function __construct($inner = NULL) {
+    $this->inner = $inner;
+  }
+
+  public function greet() {
+    if ($this->inner) {
+      return 'I am a decorator';
+    }
+    return 'I am not a decorator';
+  }
+
+}