I had a similar scenario where in I want to move the clients from one server to another server that has a newer version. Upgrading is not an option.
What I did was change the primary server in the policies page to the new one. Went to clients and right click on the group to update content.
I'm not sure if that is really working.
But what I did was use the Sylink Replacer on the PCs. The only problem here is that the PCs must be turned on during this procedure.
I'll also try Mugged's advice if I can squeeze it in. My problem is that the server does not have a backup so if everything fails. I'll have to rebuild from what's left.