Name changes are pretty often in the I.T. field with user’s getting married and for various other reasons which requires you to need to modify their UserPrincipalName.
You can change a user’s login name by selecting the user, clicking modify and click Change Login.
This will modify the user’s UserPrincipalName which will break devices from syncing. The user will need to re-enter their new username and password on all mobile devices, Outlook, and any other devices connected to your system.
If you use any sort of remote desktop services or other VDI solution then you may have roaming or redirected profiles configured. Change the user’s UserPrincipalName will break the redirected profiles and end up creating a new profile for the user.