Clients cannot AutoUpdate - New Primary Server

I'm looking to roll out Sophos Endpoint Security to 700 + clients in a school. The problem is that many of these clients already have Sophos Endpoint Security installed from a now defunct server. They are looking to autoupdate from the old server and we now have a new AutoUpdate server and Enterprise Console. When I try and manage these machines from the new server it fails. If I log on to one of these machines I have to cancel the autoupdate and uninstall Sophos and reinstall manually (it won't deploy from Enterprise Console). It would appear that because the clients are looking to autoupdate from a now invalid source, they won't listen to anything else. Any ideas?