The upgrade availability was posted in our store, on the forums, and in a few other places for quite some time. I'm sorry you missed it.
The ATi units originally just pointed at our own update servers so we could make sure nothing broke with MythTV or XBMC. After a while we realized this was probably overzealous, and just served to delay updates, so the later models just updated from Ubuntu directly.
Ubuntu has a tutorial on setting/changing the sources for updates: https://help.ubuntu.com/community/Repositories/Ubuntu
If you're still running on a flash drive, make sure you run 'apt-get clean
' from the command line between any updates, to free up space. Alternately, you can just create a clean install of Ubuntu, being careful not to install too much extra, and remove any unneeded packages.