That's really a difficult question to answer. I've done it before with no problems, but the chance of having an issue definitely exists, and you have to be prepared with a good backup of your system. I did an Ubuntu upgrade from 6.06 to 8.04 without any issues, but doesn't mean it would go smooth for everybody. I could be running on different hardware and using different drivers from the next guy. So many variables involved to give you a straight answer that it will work with no issues. When I upgraded I did the Zimbra shutdown and then did my OS Upgrade, reboot, and Zimbra started with no issues. I've since done a total rebuild and converted from 32bit to 64bit, but that's a whole different story.
Release 6.0.2_GA_1912.UBUNTU8_64 UBUNTU8_64 NETWORK edition + Mobile Option
Activesync with Moto Q9C, HTC Touch Pro, Palm Pro, & Palm Pre