I found this Wiki article below that has instructions on moving from 32 to 64 bit. I'm looking for the easiest process for migrating existing users, email, etc to a different machine of the same architecture but a newer version of RHES. My current version of Zimbra is Release 5.0.10_GA_2639.RHEL4_20081003014818. Also, the new server will have a different hostname and IP address, but our mail.domainname.com alias will point to the new server.

Network Edition: Moving from 32-bit to 64-bit Server - Zimbra :: Wiki