question

>> the main problem is that if a player plays without connecting to google play - we use their device ID to login - meaning only that specific device can connect to that specific account. we want to use google play to allow players to connect to their account from other devices. what should I do?

You don’t have to delete the old account and create a new one for google login, you can use API LinkGoogleAccount to add google as a login method to players to let them login with google account on any devices they want. For more info, please refer to Setting up PlayFab authentication using Google Play games sign-in in Unity.

>> another question - can any one think of a different/better way to approach this problem? is there any way to change a playfab username to a new one when logging in with google play?

If the user has set the username then it is not able to change it. If you want the PlayFab username same as the Google username, you should set the PlayFab username after the Google account has been linked. If you cannot change the workflow like that, and you have to delete the old account, then you can use API DeleteMasterPlayerAccount to delete the master player account or API DeletePlayer to delete the title player account. For more info about title player account and master player account, please refer to Available built-in entity types.