Observer Notifications after Transfer of Authority

Is there a way to turn on notifications after transfer of authority, for users who are observers but not approvers for a workflow? User A is away and transfer of authority on all changes has been assigned to user B. User B receives all notifications for any changes where they are the approver, but not where they are the observer. I assume it has to be some combination of criteria and notification objects, but I can't quite figure out how to make it work.