active directory – Change a synced AD user’s Username

We have a user called John Smith, his username is a local AD user called – this is synced with AD connect to microsoft 365.
We have a cloud shared email address called Where is a delegate user. is no longer a company so now needs to become a shared email address. needs to become the main username for John. Both email addresses need to continue working.

In the cloud I converted to a regular mailbox, and converted to a shared mailbox. In AD, I changed John Smith’s email address and username to – there were no proxy addresses showing in attribute editor. AD Connect reported errors and the Office 365 webpage would say the username still was After this didn’t work I also tried changing proxy addresses.