We've upgraded testing server too. Soon we'll go into production. also Centos x86_64.
Advice for backup:
1. stop zimbra
1a. check if every process is really stopped. don't want to see stalled mysql or something - happened to me once
2. tar zimbra installation folder
3. tar /var/lib/rpm !!! (I forget often to do this)
4. START ZIMBRA!!! (else upgrade could fail)
5. upgrade
6. if everything works, delete backups, else untar and try again
