When attempting to change values on a user object in 365 with DirSync enabled you are told that ‘This user is synchronized with your local Active Directory. Some details can be edited only through your local Active Directory’.
So, you fire up Active Directory, change the last name, display name, add the new SMTP proxy address, and set it as the default. The request also calls for a user name change, so you change the user logon name value to reflect the change and update the email address field on the General tab. Finally, you kick off a sync by running the command Start-OnlineCoexistenceSync on your DirSync server.
When you look in the 365 console the user login name hasn’t changed. And the old value is required to setup the phone and log in to OWA.
To fix this, you’ll need to connect to 365 via PowerShell (https://technet.microsoft.com/en-us/library/JJ984289%28v=EXCHG.150%29.aspx). Once logged in, run the following command and wait a few minutes.