Here's the explicit wording from:
https://oem.microsoft.com/script/ContentPage.aspx?pageid=552862
"Generally, you may upgrade or replace all of the hardware components on
your customer's computer and the end user may maintain the license for the
original Microsoft® OEM operating system software, with the exception of an
upgrade or replacement of the motherboard.
An upgrade of the motherboard is considered to result in a "new personal
computer" to which Microsoft® OEM operating system software cannot be
transferred from another computer. If the motherboard is upgraded or
replaced for reasons other than a defect, then a new computer has been
created and the license of new operating system software is required.
If the motherboard is replaced because it is defective, you do NOT need to
acquire a new operating system license for the PC.
The reason for this licensing rule primarily relates to the end-user license
agreement (EULA) and the support of the software covered by that EULA. The
EULA is a set of usage rights granted to the end-user by the PC manufacturer
and relates only to rights for that software as installed on that particular
PC. The System Builder is required to support the software on that
individual PC. Understanding that end users, over time, upgrade their PC
with different components, Microsoft needed to have one base component "left
standing" that would still define that original PC. Since the motherboard
contains the CPU and is the "heart and soul" of the PC, when the motherboard
is replaced (for reasons other than defect) a new PC is essentially created.
The original System Builder, therefore, can not be expected to support this
new PC that they in effect, did not manufacture."