A major hardware change (like mainboard, cpu or the boot hard drive) triggers a reactivation. However, with the new Deactivation option, explained here:
http://www.fsdreamteam.com/forum/index.php?topic=4082.0If you deactivate a product before changing the hardware, you'll reclaim that activation so, you'll be able to reinstall on a new system even if you already reached the maximum limit of 6.