First, Windows 7 is a lot more flexible with hardware changes, but at the very least, you should run Sysprep before moving to the new board. I would greatly recommend a clean install, especially if you plan ahead to cut down on how long that would take. However, Sysprep can remove some of the drivers
to allow the system to be a little cleaner.
As for the copy of Windows 7, that all depends on your license. If you have a retail license, then you are fine. It should activate on the new system without issue, and it will be legit. If you have an OEM license, then that opens the door to a whole lot of gray areas, in terms of what is considered to be a new computer, and how legit your license will be, assuming you can activate it.