diff --git a/docs/frontend-system/architecture/60-migrations.md b/docs/frontend-system/architecture/60-migrations.md
index 1c1e61e4d7..0d6ef39209 100644
--- a/docs/frontend-system/architecture/60-migrations.md
+++ b/docs/frontend-system/architecture/60-migrations.md
@@ -80,3 +80,9 @@ The "extension creator" pattern where `createExtension` was wrapped up in a func
 Replace all existing usages of extension creators in your plugin or app with the corresponding blueprint. For a given extension creator `create<Kind>Extension`, the blueprint equivalent is `<Kind>Blueprint`. There might be slight differences in the API between the two, but most often you just need to move the kind-specific options of the extension creator to be passed as parameters to the blueprint. Note that if your extension declared any additional inputs or config you'll need to use the [`.makeWithOverrides`](./23-extension-blueprints.md#creating-an-extension-from-a-blueprint-with-overrides) method of the blueprint.
 
 If your plugin exports and extension creators, these should be migrated to blueprints instead.
+
+### Extension tester rework
+
+The `createExtensionTester` from the `@backstage/frontend-test-utils` package has been reworked to better support testing of extensions. The new API allows access to extension output directly using the new `.get(ref)` method, and also provides access to all tested extensions through the new `.query(id/extension)` method.
+
+The `.render()` method that used to render the test subject in a test app has been deprecated. The extension tester no longer constructs a full app tree, but instead only instantiates the tree for the extensions under test. If you want to test the rendering of an extension that outputs in an app, you can instead use the `renderInTestApp` utility in combination with the new `.reactElement()` method of the extension tester: `renderInTestApp(tester.reactElement())`.