In Active Directory users and computers, right click Queries> New. Give it a name, click “define query”. Stay on the “Users” tab. Select “Is not” and type an invalid name like testtesttest. Click ok, ok. It should return a result with all users whose name is <<not>> testtesttest. Select all, right click> Properties. Go to Account tab> check “UPN Suffix” and change it to the appropriate suffix. That’s it! There are PowerShell methods to do this too but when handling hundreds of user accounts, you have to be absolutely careful in PowerShell. There are solutions online that work fine that I am not going to post here. This is purely for the GUI. Good luck!