Create a new service account in the legacy game's Google Cloud project, add it to the new game's IAM page, and give it the Firebase Admin role in both projects.

Create a new service account in the new game's Google Cloud project, and give necessary permissions in both projects.

Create a service account (SA) in the legacy game's Google Cloud project, add this SA in the new game's IAM page, and then give it the Firebase Admin role in both projects.

Create a service account in both projects independently and manage their roles separately.