I just attempted to create an new App Id for my paid Apple Developer account, on developer.apple.com. The Bundle Id I tried to use was already taken by my previous free Apple Developer account, so I had to use a different Bundle Id, which I did.
In my Xcode project I updated to my new Bundle Id everywhere I could find, Project settings, Info.plist and the GoogleService-Info.plist for Google SDK Authentication and Authorization. My app uses Firebase, my app still works entirely correct, I able to Login with existing account stored in Firebase Database and create new accounts that are correctly created in Firebase.
I then logged into Firebase, which I have been using with my app for a while, I created this Firebase project when I was using my free Apple Developer account. this Firebase project has my iOS app listed with the old Bundle Id:
There is no way to edit this on Firebase Bundle Id, I have to create a new Firebase App within the Firebase Project as I have read here.
To recap, I have I changed the Bundle Id in my App source code everywhere I could find to change it. I used Xcode to Clean & Build, I deleted the App from my test device and reinstalled and everything still works against the Firebase backend.
My question is: Does Firebase actually use the iOS Bundle Id for anything other than a name for an App in a Firebase Project?