Yeah you'll have to reformat, sometimes if you stay within the same chipset, or a variation of a chipset the thing will be able to load and compensate. I did manage to do this once in the intel 440 days. I inadvertantly tryed it when I built this comp. It was a intel 440bx to the nvidia nf4(amd64) chipset, it came up with an error like invalid system configuration, or something like that.
Basicly its a good idea to format/reinstall even it you could boot into windows, because some junk gets left behind by drivers.