I did this with Vista and W-7 Rc-1. I only had to re-avtivate both Operating systems. It did not require a clean install, I also had to re-activate Office 2007. My change also included a new video cardWelcome to Windows7forums..
Unfortunately no, when you change a major component like the Motherboard you'll need to reinstall your OS.. So it would be wise to do a backup of all your personal files prior to installing the new mobo/cpu..