The solution to this is posted in Doc ID 1534416.1. We just went through the same pain. The answer is to add the old ID (retrievable from the server's Info tab in the VM Manager in 3.0.3 -- not sure about other versions) to the following stanza in /etc/ovs-agent/agent.ini:
fakeuuid=<put old ID here>
It worked for us after we replaced the system board.
Edited by: user13166809 on Mar 8, 2013 11:44 AM