How do I upgrade my motherboard without reinstalling Windows 7?
Solution 1:
Windows will almost certainly install drivers for the new hardware automatically with no trouble, making the change easy. You will need to remember that substantial hardware changes will result in Windows deactivating (to prevent someone copying the disk image to many different machines). The rules by which a significant hardware change are detected are somewhat complicated but known, see here.
On never versions of Windows activation after a significant hardware change works just like activation of a new install - it will automatically attempt to activate over the internet. Internet activation will fail if the license has previously been activated too frequently*. If this happens, you will need to call the 1-800 number that Windows will give you and speak with an activation rep, who will manually give you an activation code, generally without any questions asked.
(*) for Windows XP this was more than once in the last 120 days. I believe this has changed in Vista/7, I do not know what the new rule is.
Solution 2:
There's one important thing not covered so far.
Activation.
WGA/WAT is going to complain if you change too much hardware. Nobody knows what the threshold is, but if you cross it, Microsoft support has been known to be fairly lenient in manually allowing you to re-activate regardless.
Solution 3:
Windows may boot with the new board out of the box, but it is possible that you will have to massage it go get it to work. If it does work, you should still update all your drivers from the motherboard's web site. If it doesn't the easiest way to do it is re-install and pick "upgrade": This will keep all your programs intact, but replace the hardware drivers and any other faulty registry settings. Even after the upgrade, update your drivers.
If you try to upgrade to your same OS, but are not given the option (grayed out), it is likely because your original disk is older than your current service pack level. In that case, you will need to slip-stream your original OS disk to add your service pack level or newer. You can search how to slip-stream here as it comes up often.