I think he is talking about upgrading to something other than d3/5/700it's already been proven you can.
if you have d300 or 500, a user can put a d500/d700 in there.. the things are removable/serviceable/replaceable
Which could be problematic as the Roms for the cards are on the logic Board of the nMP and not in the graphic cards themselves. Basically it would require an firmware update for the nMP itself. Would Apple allow third parties to compile their own firmware? Absolutely not. Could the rom be on the cards themselves and be forced to be initialized? Possibly. Is it worth the effort to reverse engineer that, create your own boards (2 different at that) and then sell them to a really small group of people? Not if you aren't going to force users to buy the upgraded cards for as much as a new computer...
Only Apple could ever possibly offer affordable updates