Accident on Unnassociated A

@Confused_User:

I think it should be pretty easy with these steps:

• From the "main account", ask the "now unassociated" account to rejoin. (Account Management -> Account Profile -> Link Accounts to an organization -> Invite Accounts"; see this Zoom Support page.)
• From the unassociated account, approve the request to (re-)join the account.
• In the "main account", I think the Pro account allocated to the re-joined user should still be in your available Pro accounts, and you should be able to allocate that license to the (re-)joined user.

That, or some similar process, should be all you need.