You should be able to use them again. Moving from 32 to 64 bit shouldn't affect the game. In fact I'm using Windows 7 x64 and have no problems with Witcher 2.
You should back-up your saves which are, if I'm correct, at default in your Documents map --> Witcher 2 --> gamesaves . When you've reinstalled the game, put your old saves back in that folder and it should work, no matter if 32 of 64 bit.