Change domain of existing technician

I'm preparing to migrate the server our ServiceDeskPlus is hosted on onto a new domain, but I'm having an issue with the technician accounts.

The move (on a VM copy of the server) is successful, I've added the new domain and successfully scanned it, it's even got my user account associated with the new domain, so I get single sign on as a technician, but all the other technicians are showing as associated with the old domain.

I don't want to create new logins and move cases over if I can avoid it, I just want to change the domain associated with the Technician account. It lets me change the domain to None and put it back in the original domain.

All I want to do is tell it that the other Technician accounts should look for the username on the new domain instead of the old one, but when I try, I get a box saying

"Unable to save technician : Login with same name already exists. Please provide different value for Login Name."

Just to make things really interesting, if I Import Requesters from Active Directory and point it to the old domain, it switches my account back to looking at the old domain. If I then do it again but point it to the new domain, my account gets switched back to the new domain, but my Boss' account (which is in the same OU as mine on both domains) remains pointing at the old domain.

Does anyone have any suggestions?

Thanks!