I've not tried changing databases, fortunately I think I created all of them from the appropriate client.
Not found any problems with changing things around manually. I created a dummy "test" record form the interface while logged in as that client then copied the user id and group id from that.
Just noticed that with the mailbox records, if you open and close them in the interface after changing the zone client id they appear in the clients control panel. It's also worth noting that, after the zone has been changed to a new client, creating aliases etc.. for that zone from admin assigns them to the correct client user/group.