We have a user called John Smith, his username is a local AD user called John.Smith@alpha.com – this is synced with AD connect to microsoft 365.
We have a cloud shared email address called John.Smith@bravo.com. Where John.Smith@alpha.com is a delegate user.
Alpha.com is no longer a company so John.Smith@alpha.com now needs to become a shared email address. John.Smith@bravo.com needs to become the main username for John. Both email addresses need to continue working.
In the cloud I converted John.Smith@bravo.com to a regular mailbox, and converted John.Smith@alpha.com to a shared mailbox. In AD, I changed John Smith’s email address and username to John.Smith@bravo.com – there were no proxy addresses showing in attribute editor. AD Connect reported errors and the Office 365 webpage would say the username still was John.Smith@alpha.com. After this didn’t work I also tried changing proxy addresses.