As always seems to be the case with Zimbra, all went smoothly in the upgrade.

Upgraded on CentOS 5.4_32 then migrated to CentOS 5.8_64.

Utilised the following for the migration :

Platform and OS Independent ZCS to ZCS Migration Using Rsync - Zimbra :: Wiki

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

Only minor issue on Cents5.8 had to stop and disable syslog and enable and start rsyslog to get zimbra logging working.