Short: Don't even think about it.
Fuck it. I tried to migrate several Ubuntu Hardy maschines (XEN guests) in the past view months. Since, yeah - you know: it's an unsupported LTS. The first broke due to an interupted SSH connection. Yeap, they warned me. My fault.
Then I got smarter, and upgraded trough a SSH connection to the XEN host which then connected (somehow) to serial port. Upgrade seemed (always) to go fine. Took a while and a lot of "NO dont modify my config file". In 4 cases the upgrade went through, requested a reboot and was gone for good. In 2 cases it stopped at "copying image-something from somewhere" (I guess the XEN host tried to get the linux image).
Screw it, I'll keep running Ubuntu 8-something Hardy for good. I guess backing-up, reinstalling and reconfiguring the whole thing would go a lot faster and safer.
Is this just a problem for me, or a bonkers (Xen?)-Setup (at Strato)? Don't get me wrong, I quite enjoy their (recently abandoned) Multiservers. Got three of them w/ 21 systems running perfectly smooth.