If you buy a computer with OEM Windows you cannot upgrade the Motherboard and keep the Windows Licence. You will must to buy a new Windows Licence.
Microsoft tell us:
“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.”
The reason Microsoft gave for this term is that “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.”
Microsoft sent a memo to its OEM partners asking them to enforce this new policy, every time they upgrade a computer for a client.