In general, it’s very easy to convert a user mailbox into a Shared Mailbox in Microsoft 365 / Exchange Online. Except, if you have directory synchronisation configured (e.g. AD Sync, Microsoft Entra Connect) and then it’s not. The option to do this is not presented to you in the admin console web interface.

Fortunately, it’s a quick one-liner in PowerShell:

Set-Mailbox -Identity [email protected] -Type Shared

Done. Oh, and don’t forget to go in and yank the licence from the Shared Mailbox if you don’t need it assigned any more.