I have a customer that is planning to replace their exchange 2003 server which is also a Unity 4.2.1 Partner server in a UM environment. They are going to use the disaster recovery switch as defined per this link: http://technet.microsoft.com/en-us/library/bb124429.aspx
At the end of the recovery the Exchange server will have the same IP and Hostname as the old Exchange. I don't believe that this will cause any issues for Unity assuming service packs are all the same on the new Exchange server. It should use UMR during the recovery and connect (may or may not need a restart) once the new exchange is on line.
I wanted to bounce this off the experts before feeling comfortable though.
Any feedback would be appreciated.