Possible to retrofit current iMac with next gen GPU?

It's theoretically possible but the parts are extremely hard to find and pricey plus you don't know does it fit there, let alone does your Mac's firmware support it. So, in real world, no, it's not really possible

Yeah getting a graphics driver to fully support the new hardware would be the hard part so the answer is no. Should have gone with a Mac pro if you wanted expandability

Pretty sure the only things iMacs can be expanded apon is hard drive space, ram, and possibly CPU since it uses a desktop motherboard(I think)

All mxm cards are pretty much the same size and universal, so it's very possible up upgrade the gpu-- but very tedious work to actually tear the iMac apart enough to flip the motherboard around and replace the gpu's daughterboard.

Finding other mxm cards on the market is rather hard, but nothing's impossible. I found a stock mxm 4670 that was the same one used in the iMacs rather quickly through a google search, somebody was selling it on ebay.

Graphics drivers are the biggest issue for osx gpus. If apple provides a universal graphics driver update to osx for future iMacs that carry mobility 58** cards, it might be possible to swap your older mxm card with a newer mobility 58** mxm card. Mac pro users found out they could make nvidia 480s work on the mac pro with the embedded graphics drivers apple had in some versions of osx, so there's always possibilities on the driver front.

I just gave the sunniest of scenarios though, I would not recommend trying it.

My best advice is to just buy the computer that you need from the beginning.

I used to come across a lot of customers who would be cheap and try to save a few dollars buying the crippled sale model of a computer only to a few months (in one case a few weeks) later return and complain, pay restocking fees, or in some cases just be stuck with a computer that didn't do what they needed.

If you think you will be gaming in the next 3 years, just buy the better (or the best available) GPU option from the start. Don't be cheap on that. I guarantee that everyone who claims they can't spare the 200 or 300 for a better computer that they'll use for the next 3 years wastes much more than that on eating out, going out, silly purchases, etc. That they definitley did not need without blinking an eye.