If you choose not to format, Windows may not boot after making the hardware change. You can do a "repair" install by booting from the CD. You'll be prompted at one point to actually repair the installation or install Windows again. You'll actually want to go with the second option. Confusing, I know
. I went from AMD to Intel flawlessly, but I had to repair install when upgrading later on from an Intel reference i865 to an Asus P4P800 (also an i865), so there's really no reliable metric.