with only cpu, memory, video and sound(if not using onboard) cards are put in another case with a totally different board you have simply assembled a new system altogether. That's still going to be a major hardware change past the limit allowed when Microsoft anticipated that there would be some upgrades to different hardwares over a period of time.
When you start swapping boards or go over 3 items within a certain period of time you have to reactivate anyways. If you are forced to reinstall Windows due to a need to wipe a drive due to virus or something else what do you do? call MS. It isn't that hard to do. When you swap too many things you will want a clean install anyways. It saves alot of headaches when problems come up later.
