I succesfully integrated additionally „Play as Guest“ by simply signing in the player automatically with his device (anonymous login with ios and android device-id) in my Unity Game. As soon as the player starts the game next time, the last sign-in type (Guest or Registered Player) is saved in player prefs and the player logs in automatically.
I also added the feature that the „Guest“ can register afterwards and turn his account into a „full“ account by adding Username, E-Mail and Password. I don‘t allow Guests to make Real-Money purchases unless they have registered.
But there‘s one thing I didn‘t yet understand. What if an Androidplayer Logs in with his Device-ID, adds an E-Mail + Password, deletes the game from his device and then sells his phone? Let‘s say the other guy who gets the old phone also instals my game and presses „Play as Guest“ - won‘t he then log into the previous phone‘s user‘s account, even if he doesn‘t know his E-Mail and Password, and have also access to purchased items and the full account? How would I avoid that this can happen? Do I need to unlink devices after Email + Password were added and if yes, how do I do this?
Thanks for your help and sorry if I‘m asking dumb questions.