Change Office 365 sign-in name for AD synced user

To change Office 365 sign-in (Principal) name for user synced with Onsite AD:

1. Start PowerShell
2. Make sure the Microsoft Online module is loaded
3. Execute the following command:

set-msoluserprincipalname -newuserprincipalname newuser@emailaddress.com -userprincipalname user@emailaddress.com.

Source: msonlinehelpdesk.zendesk.com