diff --git a/packages/core-compat-api/src/convertLegacyApp.ts b/packages/core-compat-api/src/convertLegacyApp.ts
new file mode 100644
index 0000000000..9343744075
--- /dev/null
+++ b/packages/core-compat-api/src/convertLegacyApp.ts
@@ -0,0 +1,138 @@
+/*
+ * Copyright 2023 The Backstage Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import React, {
+  Children,
+  Fragment,
+  ReactElement,
+  ReactNode,
+  isValidElement,
+} from 'react';
+import {
+  BackstagePlugin,
+  ExtensionOverrides,
+  coreExtensionData,
+  createExtension,
+  createExtensionInput,
+  createExtensionOverrides,
+} from '@backstage/frontend-plugin-api';
+import { getComponentData } from '@backstage/core-plugin-api';
+import { collectLegacyRoutes } from './collectLegacyRoutes';
+
+function selectChildren(
+  rootNode: ReactNode,
+  selector?: (element: ReactElement<{ children?: ReactNode }>) => boolean,
+  strictError?: string,
+): Array<ReactElement<{ children?: ReactNode }>> {
+  return Children.toArray(rootNode).flatMap(node => {
+    if (!isValidElement<{ children?: ReactNode }>(node)) {
+      return [];
+    }
+
+    if (node.type === Fragment) {
+      return selectChildren(node.props.children, selector, strictError);
+    }
+
+    if (selector === undefined || selector(node)) {
+      return [node];
+    }
+
+    if (strictError) {
+      throw new Error(strictError);
+    }
+
+    return selectChildren(node.props.children, selector, strictError);
+  });
+}
+
+export function convertLegacyApp(
+  rootElement: React.JSX.Element,
+): (ExtensionOverrides | BackstagePlugin)[] {
+  const appRouterEls = selectChildren(
+    rootElement,
+    el => getComponentData(el, 'core.type') === 'AppRouter',
+  );
+  if (appRouterEls.length !== 1) {
+    throw new Error(
+      "Failed to convert legacy app, AppRouter element could not been found. Make sure it's at the top level of the App element tree",
+    );
+  }
+
+  const rootEls = selectChildren(
+    appRouterEls[0].props.children,
+    el =>
+      Boolean(el.props.children) &&
+      selectChildren(
+        el.props.children,
+        innerEl => getComponentData(innerEl, 'core.type') === 'FlatRoutes',
+      ).length === 1,
+  );
+  if (rootEls.length !== 1) {
+    throw new Error(
+      "Failed to convert legacy app, Root element containing FlatRoutes could not been found. Make sure it's within the AppRouter element of the App element tree",
+    );
+  }
+  const [rootEl] = rootEls;
+
+  const routesEls = selectChildren(
+    rootEls[0].props.children,
+    el => getComponentData(el, 'core.type') === 'FlatRoutes',
+  );
+  if (routesEls.length !== 1) {
+    throw new Error(
+      'Unexpectedly failed to find FlatRoutes in app element tree',
+    );
+  }
+  const [routesEl] = routesEls;
+
+  const CoreLayoutOverride = createExtension({
+    id: 'core.layout',
+    attachTo: { id: 'core', input: 'root' },
+    inputs: {
+      content: createExtensionInput(
+        {
+          element: coreExtensionData.reactElement,
+        },
+        { singleton: true },
+      ),
+    },
+    output: {
+      element: coreExtensionData.reactElement,
+    },
+    factory({ bind, inputs }) {
+      // Clone the root element, this replaces the FlatRoutes declared in the app with out content input
+      bind({
+        element: React.cloneElement(rootEl, undefined, inputs.content.element),
+      });
+    },
+  });
+  const CoreNavOverride = createExtension({
+    id: 'core.nav',
+    attachTo: { id: 'core.layout', input: 'nav' },
+    output: {},
+    factory() {},
+    disabled: true,
+  });
+
+  const collectedRoutes = collectLegacyRoutes(routesEl);
+
+  return [
+    ...collectedRoutes,
+    createExtensionOverrides({
+      extensions: [CoreLayoutOverride, CoreNavOverride],
+    }),
+  ];
+}
diff --git a/packages/core-compat-api/src/index.ts b/packages/core-compat-api/src/index.ts
index 2181b2fa9a..e5f61119a3 100644
--- a/packages/core-compat-api/src/index.ts
+++ b/packages/core-compat-api/src/index.ts
@@ -14,3 +14,4 @@
  * limitations under the License.
  */
 export { collectLegacyRoutes } from './collectLegacyRoutes';
+export { convertLegacyApp } from './convertLegacyApp';