windows 7 32 Bit - Ghosts 64 Bit

HI need some help, blops2 runs fine on my computer but with Ghosts coming and Ive pre ordered I see it will only run on 64 bit. My pc is 64 bit compatable. Does this meen I'll have to change it to 64 and if so how do I do it without losing any of my stuff on my PC.

Thanks