Yes. Motherboards are considered major changes. Because if you replace the motherboard with a better one than the original manufacturer's part, you are effectivley creating a new computer and therefore, if your Windows license is an OEM license, would be in violation of the OEM license terms.
Simple really.
Not always that simple. I build my own PCs these days, and I don't recall ever seeing the same assortment of motherboards at the computer store when it's time to replace a bad motherboard. You're essentially forced to use a better motherboard even if you just want equivalency. And often that forces you to replace a lot of other stuff as well.