Keep user account but provision new empty mailbox

i did ask in another forum but thought i would ask here as it seems impossible...

we are hybrid exchange. We have litigation hold and purview retention policies in place. We have a scenario where an existing user is moving to a new role and her existing mailbox needs to be dissociated from her AD account and a new clean mailbox provisioned. The original mailbox needs to stay as inactive and searchable via ediscovery.

Is it possible? I have asked AI and its said:

1. Make sure all the holds and retention policies are in place
2. Move the AD account to a non-syncing OU and run a delta sync
3. The mailbox should show as inactive in exchange online
4. Then it tells me to run Set-User <UserUPN> -PermanentlyClearPreviousMailboxInfo but ONLY if the recipient type shows as MailUser or User

This is where i am stuck as it is still UserMailbox. It told me to restore the cloud only object which i did. But it still shows as RecipientType = UserMailbox when i check. Its now just a cloud only account, it has no license. The mailbox is inactive but its still a UserMailbox

Is what i am trying to do possible? Would now just changing the cloud only account to have a new email address be the only way to retain it and then sync back the on-prem account?