Just do it via PowerShell:
Set-MsolUserPrincipalName -UserPrincipalName email@example.com -NewUserPrincipalName firstname.lastname@example.org
So, we've migrated users from on-premise exchange to office365 & everything's working fine. However, the usernames of the users is in the format "abc@xyz .onmicrosoft.com" while the primary smtp email address is email@example.com"
We tried chanding the username from the active user's section but faced the follwoing error; "This email address is already in use by an active USER. Try a different email"
The email address is used by the user itself (no other user)
Please, share your feedback on how to resolve this issue - make the username same as the primary email address.