Just keep the Windows 7 installation on the HD and allow it to swap out the drivers
for the new mobo when it starts up. It will ask for several restarts in the process.
I have done this quite a few times with no adverse effects.
Then use the link at Computer>Properties to refresh (change) the product key for the new hardware. It might require a robocall to MS to trade a series of numbers that deactivate the OS on the old hardware and activate the new.
If there are any performance issues, then you can run a Repair Install
to acclimate the OS to the new hardware. This requires reactivation afterwards at Computer>properties.
Finally, you have the option to do the cleanest possible install which is to boot from the DVD, select Custom install, use Advanced drive tools to format before install.
Be sure to back up your files.