Allowing an App Engine app to access another App Engine app's datastore

I have a situation where an existing GAE App (let's call it app A) is running, but for non-technical reasons can't be modified. As users migrates to a new client version, we need to migrate their data from app A to a new GAE app (which I'll call app B).

Is there a way that I can grant app B access to app A's live datastore without modifying app A? My not modifying I mean not having to deploy new code. Changing setting or permissions in the Cloud Console is fine.

In case it matters, both apps that I'm referring to are written in Go.

It might not be possible to share the datastore across multiple GAE apps using the Google App Engine Standard Environment Client Libraries. At least for python it's not possible, donno about go.

But the Cloud Datastore Client Libraries can be used to share a datastore across many apps, even from outside Google Cloud.

Regardless of the particular way the old app accesses the datastore (language/library/etc.) it can be configured from the Cloud Console to allow access to a remote app. The exact procedure steps are captured in https://stackoverflow.com/questions/42051453/how-do-i-use-google-datastore-for-my-web-app-which-is-not-hosted-in-google-app-e/42060551#42060551

The new app would be using the above-metioned client library with the old app's service account credentials (obtained in the above paragraph procedure) to access the old app's datastore.