Did you update and extend the schema on your DC’s first? Can you demote the exchange server and remove the DC role from it? It is against microsoft best practices to have exchange and dc roles on the same server anyhow. I would try removing the dc role first from that exchange server. Our organization migrated from exchange 2000 to exchange 2007 this year. We had exchange 2000, 2003 and the new 2007 exchange servers all in coexistence for just over a year until the mgration was complete so this is possible. We had to have all of our DC’s at the same level at each stage however. We updated them to server 2003 DC’s first before installing the 2003 exchange server, then updated them again before installing the exchange 2007 servers, but we did not remove any of the exchange servers until we were completely migrated over to 2007.