I just wrote a script that I used to migrate from 32-bit Centos4 to 64-bit Centos 5. It will probably work for other platform migrations too.
I uploaded it to the Wiki.
Platform migration script - Zimbra :: Wiki
I also since discovered an "official" 32-bit to 64-bit migration page here which is probably more tried and tested but is a manual operation and pre-Jetty.
Network Edition: Moving from 32-bit to 64-bit Server - Zimbra :: Wiki