Welcome to the wonderful world of Microsoft.
There are "legal" ways to put off activiation for a year and both legal and
illegal ways to generate a valid license key.
If you upgrade a video card you are told your license is invalid, let alone
a motherboard. Vista did that to me, but I don't give a rat's behind because
I only have Vista on a dual boot system as it is totally unusable.
Alas you must call the Microsoft center in India, Microsoft being dedicated
to keeping jobs in America their largest market-not, and waste ten minutes
getting a new set of numbers read to you, after swearing on all that is holy
or unholy to you that you are not trying to cheat Microsoft out of foisting
off another copy of Vista.
When all is said and done, however, you will still be running Vista and no
motherboard replacement can change that, activated or not.
Sorry for the bad news.