we're most likely going to be changing our AD domain name, and was trying to look at the impact on Unity 4.2.1 unified messaging integrated with Exchange 2003. I know with the domain I'd use rendom, and then have to use the XDR-fixup tool to fix exchange. it does not appear to change the Exchange Org name. I suspect the answer is DIRT and rebuild for it to work, but I'd rather not rebuild all 3 unity servers.
Yes, you will need to do a DiRT backup and restore. You are essentially moving Unity to a new domain which is described in the following link.
Hope this helps. If so, please rate the post.