Just brought mine from Amazon for £12.47 with free p&p which i thought was a bit of bargin as i have less that half of the DLC available for F3 and that after selling my F3 i only had to find £5 to upgrade with.
As for my career, well after having a HDD problem a few weeks ago and not backing properly before formatting i have had to start right from the start anyway. I am hoping that the game save file might work with new version, i will let you know how i get on once it arrives.
Game save does carry over ok. I updated my install using the below guide. After installation you can play using either your original Forza disk or the ultimate disk depending on which disk 1 you have installed. The game will play from either disk but will only load from the HDD if the disk version matches. If they don't match the game runs from the DVD.