Hmm....you will get a windows reminder about the registration in 3 or 4 days, thats what happened to me when I switched from AMD to intel a few months back. If you're using different chipsets and such, you probably will either have to hunt down all the old ones and delete them, install new ones etc...but with the way stuff like that likes to hide things in the registry, youre going to be chasing gremlins I would say and thats just a pain.
I can do a complete reformat and OS install back to the same exact point I am now in between 2 and 3 hours. I just have an "essential install" folder of all the stuff that I must have in windows, I have all my keys written down, and boom. All I have to wait on then is windows to re-download the updates. That is always the best bet and as long as youre data is backed up, youve got nothing to lose. Just make sure youve got your keys handy before you delete all that stuff, I got all my ducks in a row with the different companies to find all my missing keys before I did my first one, and since then Ive done it about 6 or 8 times.
I will be doing this same thing this week when I move my storage HD and 64gb m4 onto one 256gb m4. I didn't want to make a system image of my o.s. drive because I do have dome things installed on my storage drive, and I didnt want windows to act wierd while its searching for those little file remnants on a drive that doesnt exist anymore.