I don't have a Sky+ box, so can't comment on upgrading details .....
(but plenty of people do, and no doubt will soon be offereing help!).
However, there are useful weblinks.
For example, there's complete "risk free" (!) kits:
www.tempodigitalmedia.com/default.aspx?id=1 .
(but it's probably much cheaper to buy the bits yourself).
Googling "sky+ upgrade" will get other stuff (including advice on other satellite forums, if I'm allowed to say that!), etc .......
I've found this upgrade guide at digitalspy, Rolf, please forgive .......
http://forum.digitalspy.co.uk/board/showthread.php?t=156344 .