There’s been a long-standing issue with Office 365 where you cannot change a user’s userPrincipalName (UPN) from one federated domain to another.  When attempting to make a UPN change between two federated domains, the tenant’s technical contact will receive a “Directory Synchronization Error Report” email which contains the new userPrincipalName and the user’s immutableID.

 

In order to remediate the issue, we need the user’s old userPrincipalName; this script helps with getting the proper value and making the necessary change.
Additional information on this script is available at: Office 365 – Script To Change UPN Between Federated Domains