One means that it will try to match users up every Synchronisation in EV. If all users are now on Exch2007, set the key to 1, sync everyone, and delete the key. As long as you have decommissioned the Exch2003 server I dont think EV will try to relink back to that but you never know your "luck"
Stick a solution flag somewhere if you are happy with the post is resolved please :)