Not authoratitive, but based on my experience and a little reading:
The Windows validation process uses a fair number of data items such as CPU id, drive serial numbers, and stuff that I don't know about to see if your system has changed. It tolerates a small number of changes (for example, I have swapped system drives with no problems).
One thing that I am sure if: If it barfs due to the new mobo, Microsoft will allow you to revalidate the new system.