How to convert a mailbox to be a shared mailbox, and remove their AD (Active Directory) account

 If you want to convert a mailbox to shared to preserve it, but you want to disable or delete the AD account, do the following-

 - Go into mailboxes in Exchange Admin Center, click "Convert to Shared" on the right.

 - To to the users in Microsoft 365, remove the users license.

 - Disable or delete the AD account, wait for Azure AD to sync.

 - Under Admin Centers, click Azure Active Directory, Users, Deleted Users.  Select the users and click "Restore Users."  This will move them to a "Cloud User" which will need to be maintained to prevent the mailbox from being deleted.  This account will not be able to log into the mailbox, however, so it presents a pretty minimal risk.  You can set a complex password and enable MFA.