MS does support re-activation of OEM under appropriate reasons. But, they
get to define what is appropriate.
MS has stated replacing a motherboard is an upgrade that violates the EULA.
(Whether you agree or not is irrelelvant.)
I stated, "...MS may state that you will have to purchase a new license."
MS cannot prevent you from attempting to install their OEM product on the
wrong computer. Hence, an activation process. In addition, they have put in
place a means by which the system builder is protected from this process
called BIOS lock or SLP. This process defines the term "single computer" in
the EULA. Should you attempt to install a BIOS locked version on the wrong
computer, it fails. If you change out the motherboard with a BIOS locked OS,
it will fail. This protects the system builder from being responsible for
users who move their OS to other computers. And a boundary, or definition is
established for the EULA.
In the event, you are successful at installing a new motherboard and the OS,
then activation comes into play. Auto-activation for OEM will fail. You must
talk to a representative. And it is here, no matter what you want to believe
you understand about the EULA, that MS can dictate what their EULA means
with one simple phrase: "Per the EULA, please consult with your system
builder."
The original system builder will send you a recovery disk. If it is not the
original equipment, then the recovery disk will fail.
If you are the system builder, then you can plead your case with MS, but
they don't care. As far as they are concerned, they met the letter of their
agreement. Good luck.