more food for thought, try going to an Apple Store and buy an Airport card for your 2009 cMP. They won't sell it to you.
The only way to buy one is to pay them to install it. (Or aftermarket) they don't consider an Airport card a "user serviceable part". So you have to lug the entire machine to them to put in 2 screws and connect the antenas.
And as we now know, to place a new GPU in 6,1 you would have to remove 2 out of 3 of the main PCBs, AFTER running a firmware update. If they don't trust you to put an airport card in, does anyone truly think they'll hand you a box with new GPUs and say "Good Luck with the firmware flash , hope you don't brick it"? Of course not.
We expect 7,1 to contain new tech, possibly TB3 or DP 1.3. And both of those things run through those PCBs. Is it likely that Apple is going to task the engineers with keeping new parts compatible with 6,1 so they can be used there too? Does that sound at all like Apple?
Of course not. TB3 is different tech with a different connector.
The only chance of Apple producing new GPUs that work in 6,1 is if the 7,1 is a "warm over" like 5,1 was to 4,1. Faster SSD and new GPUs but no other substantive changes. Then, maybe if they create a boot rom that flashes on to 6,1 AND sell GPUs outside of service lines, then maybe for a few thousand dollars you will have an upgrade if you bring it in to the Apple Store and let them do it. But that is a long string of "maybes" and also requires 7,1 to be a rather stagnant product. i don't see it happening.
Look at all the other Macs which have lost upgradeable parts. The trend is clear and all in one direction. And as I pointed out at the beginning, if Apple had intended for GPUs to differ from the originals, they could have spent $1 extra per machine to include EEPROM chips on both GPUs. It literally would have cost them $1.
In all honesty, I would guess that 7,1 will have altered connectors if in same form factor. Add or remove some pins so there is no way to intermingle parts.