The MAC address is the address of the network card, which can be either built into the motherboard, or on a replaceable card… so if that was the only thing they tracked, you could replace everything except that… unless you have a network card with an editable MAC (they don’t need to be unique worldwide, only on the network they directly connect to).
Microsoft seems to use a slightly different system, where they’d generate a sort of hash for all the components, then allow a limited number of changes per year, so you can change the while computer a limited number of times a year… but they call home all the time.