I recently had an issue in an environment where Office 365 was being synced with AD and a user that was renamed in AD was not getting fully synced over to the cloud. Office 365 was picking up the new name and updated email address but not the “User Name”.
You are not able to modify this field using the GUI interface, but fortunately you can manually set this field using the PowerShell command below.
Set-MsolUserPrincipalName -UserPrincipalName [CurrentUPN] -NewUserPrincipalName [NewUPN]
More details at http://support.microsoft.com/kb/2523192