How to change Domain for Technician

Hi,

We are using build 7605. We are in the process of migrating staff from one active directory forest to a new active directory forest. Once a person has been migrated (account/workstation/mailbox) I need to be able to change the Domain that the technician uses to login to SDP. However, I am unable to do this. I have admin. priviledges and when I open a Technician to edit, and select the new domain from the Domain drop down list in the Login Details section and then click Save, it does not save the changes and just reverts back to the original selection from that drop down list.

I've tried the following:

The new domain and the old domain are setup in Windows Domain Scan
I have initiated a scan of the new domain, and it starts but never seems to complete. i can let it run overnight and it is still going the next day. the system seems to authenticate OK to active directory as I can see the list of OUs and select the ones I want to scan (i.e. that contain the user accounts)
We do not use Pass Through Authentication
The tecnicians have been using SDP successfully in the old domain for some time
Strangely, I have successfully changed the domain on one Technician only, but cannot change the domain on any other technicians, even though their configurations look to be the same

Thanks
Paul