I successfully migrate and upgrade zimbra
First migrated zimbra Release 6.0.10_GA 32bit on ubuntu 8 LTS to Zimbra Release 6.0.10_GA on Ubuntu 10.04 LTS successfully
then upgraded Zimbra 6.0.10_GA 64 Bit on ubuntu 10 LTS to Zimbra 7 on ubuntu 10.04 LTS
so far [apart all other function] the best and noticeable improvement i have seen is system utilization , i.e. CPU and memory.
I will give a applause to zimbra team for this improvement