For starters its not as simple as it sounds.. took me a whole weekend to do the migration on one box..... Basically what i updated my zimbra to 8.0 on the 10.04 box.... backed up my /opt/zimbra folder to an external drive using " # rsync -avHK /opt/zimbra /media/Passport/zimbra."... /media/Passport was my external 500GB drive.. I installed a new hard drive into the server and did a fresh install of ubuntu 12.04 lts.. applied all updates to it... Then installed zimbra 8.0 on it. Once the installation was done and i could log into the admin console via the browser... I synced back the backup i made to the external drive by "rsync -avHK /media/Passport/zimbra.bak /opt/zimbra" ...the sync took a few hours.. once the sync was complete.. i ran the zimbra 8.0 for ubuntu 12.04lts again... and it asked me if i was doing and upgrade i said yes.... when it reached the mysql check, i told it not to do it.. and just let the upgrade continue and that was it...
The reason why i used a new drive was basically i didnt want to be in a position where my email server would be down on the following Monday... and also the upgrading of Ubuntu 10.04 lts to Ubuntu 12.04lts via the package manager never when smooth enough for me .
I hope this helps.
With Jah Jah
Anything is Possible