When changing hardware such as motherboards I would always recommend a fresh install, it will give you the most stable system in the long run.
Remember to back up everything, and make sure you have all the installers and keys for your software. nirsoft has some great utilities to grab your current windows and office keys (easier than finding them in the box of junk we all have).
On windows 7 - If you really can't do a fresh install I would take a look at the sysprep command. You can do a quick and dirty sysprep where you basically sysprep -generalize then shutdown change the hardware then start it up. the windows install will freak out, and not boot. Then restart in safe mode (Hammer F8 whilst starting up) and it should start redetecting hardware. Then reboot and you are good to go. I have used this successfully a few times when in a pinch, but do so at your own risk. You will need to reinstall all the drivers again but your software should all work.
On windows 8 I havent had to do this so I cannot verify success
In the end as long as you have a full system image backup or an Acronis or other clone you can do what you want and restore if it all goes wrong.
Oh and in case I didn't make it obvious enough - BACKUP EVERYTHING
EDIT: when changing hardware configuration it is likely you will have to reactivate windows too!
Edited by bobthewonderdog, 09 January 2015 - 06:08 AM.