The 'problem' with Mac laptops is that generally they are not upgradable except for RAM and HD/SSD and sometimes the battery. And newer Mac laptops none of the above.
At one time I had heard of Mac laptops with video cards that were on a daughter card of sorts. So theoretically replaceable but no one ever made a replacement to my knowledge.
This is becoming more and more common to find laptops from other manufacturers where there is little, if anything to upgrade. It is (probably?) a side-effect of the push for smaller/lighter making modular parts less and less of a reasonable (from an engineering/design standpoint) option.
So essentially what Tetsujin is correct: you have no upgrade path for system boards, CPUs and the like.