I was working on a client's laptop, on which I'd installed Ubuntu 9.04 a few years ago and it hadn't been updated since then.

I wanted to bring it up to date, and knew that the client wouldn't want the look-and-feel of it to change radically, so to avoid Unity and Gnome 3, I decided to install Xubuntu 11.10.

I backed everything up, expecting to do a clean install of the new system.  But the Xubuntu installer offered to upgrade directly from 9.04, so I tried it, and it worked! 

The only minor glitch was that Thunderbird didn't find the old profile.  I had to rename ~/.mozilla-thunderbird to ~/.thunderbird, and then that worked too.