HI, Klllerii, I can share with you what I know about swap vga card in G750 series. I recently change from 770m to 780m so I upgrade my JX to JH, this change needed other parts to be replaced (bottom case, vga heatsink and fan, AC brick). What I learn after doing this upgrade is the following.

1.- G750 laptop series use a custom PCB form in the VGA card, so you can´t just swap it with other card from other brand. (this will require mod the heat pipes and heatsink to fit the case)
2.- All G750 series use the same PCB form so it will fit with the right heatsink. There are 3 different form heatsinks (1 for JW/JM/JX, 1 for JS and 1 for JH, and for JZ/JY, same form as JH but is made of copper instead of aluminium found in JH model).
3.-The 980M found in JY model will fit inside your JH and you can use your VGA heatsink. This no means it will work since no one have tried this already, you will need to do some bios mod, the JH BIOS does´t like other VGA cards, but you can use JX BIOS (I installed the 780m using JX BIOS and all work ok so JX BIOS seems more compatible).
4.- In order to find the 980M for JY model you can use eBay.

Good luck