Ez a megoldás ("lapzárta" után Googliztam... :)
Upgrading from Ubuntu 10.04 LTS to Ubuntu 12.04 LTS
To upgrade from 10.04 LTS on a desktop system before then, upgrade over the network with the following procedure.
Start System/Administration/Software Sources
On the Updates tab, set Show new distribution releases: to Long term support releases only, then press Close.
Press Alt-F2 and type update-manager -d
Click the Check button to check for new updates. If there are any updates to install, use the Install Updates button to install them, and press Check again after that is complete.
A message will appear informing you of the availability of the new release. Click Upgrade.
Follow the on-screen instructions.
Ebből ugy az következik, hogy nem elégséges a Frissítéskezelőben a hosszú idejű szupport beállítása, hanem utána le kell futtatni az update managert -d kapcsolóval.