I'd concur with p24t, two machines is the best route. I'd recommend to copy off all of your Zimbra files and database to another system or backup solution, then wipe your main system clean. While you can update distros between major versions, it has never been recommended as things change can radically.
Mail | Dell 2950III | 2x Quad Core 5420 | 8gb RAM | 6x 146gb SAS RAID 0+1 | Red Hat 5.3 | Zimbra 6.0.10 Network Edition
Test | VMware ESXi Whitebox | Phenom II Black 3.2ghz | 12gb RAM | 6x 1tb SATA RAID 0+1 | CentOS 5.4 | FOSS, Not in use now