they really don't tell people how they identify the hardware signature. It is a trade secret I think and will prevent people from hacking it. people have tried what you said but you then have to use phone activation and tell them you had to replace the motherboard. I don't think the support people would know how the signature is calculated either. You call them, they querry a database and read from a help script if they don't know what to do. I am pretty sure the key words will be "I had to replace my motherboard and my windows will not activate" most support people will then check your key to see if it is a banned key (pirate) and just generate a new one for you.
overall, I think it is more important for microsoft to get rid of all of the old windows versions just so they can reduce support costs
(not only for microsoft but the entire world that uses a microsoft product., development cost of many versions of drivers, on many different versions of windows each with there own set of bugs, then the cost of support people with different levels of expertise)
windows looks pretty bleak, when you have OEMs that will not provide drivers to microsoft so they can be updated with out the user starting the process by knowing they have to look for the driver updates since the manufacture will not give them to microsoft
(or can not give them because of certain bugs) Microsoft used to charge $50,000 to take a OEM driver, many OEMs refused. Now I don't think microsoft charges anything but they make you say that you ran some tests. OEMs still refuse to provide the updates. (I suspect their driver fail to pass the tests)
TJ Hooker :
@johnbl do you know if it matters how much hardware is changed? For instance, could you activate it on one computer, and then sometime down the road install Windows using the same license on a completely different computer (as long as it's no longer being used on the original computer)?
Not really related to the OP's question, just curious.