Accidentally changed UPN of Directory sync service account on Office 365 

set-msoluserprincipalname -userprincipalname SyncAccount@domain.net  
-newuserprincipalname SyncAccount@domain.onmicrosoft.com 

From <https://serverfault.com/questions/768123/accidentally-changed-upn-of-directory-sync-service-account-on-office-365>