As per this thread:
[SOLVED] Migrate Zimbra from Ubuntu 32bit -> Ubuntu 64bit
...I upgraded to 64bit.
Now that there's a non-beta version of Zimbra for Ubuntu 10.04, I would like to change my Zimbra install from "zcs-6.0.8_GA_2661.DEBIAN5_64" to "zcs-7.1.0_GA_3140.UBUNTU10_64".
What is the recommended method of doing this?