Mid 2011 iMac 27 inch GPU / Graphics card change to metal compatible so Mojave can be installed

Not possible. There are no "cards" that work in an pre-Late 2012 iMac that are Metal-capable. The Metal-capable iMacs don't have cards any more--the video hardware is soldered to and integral with the logic board and cannot be transplanted anywhere. The vido hardware now looks like this:

[image from iFixit.com]

The little "specks" you can see through the heatsink are all the video hardware.

I've been reading about some third party compatible GPUs

Those only work in certain older Mac Pros, not iMacs.

If you are running High Sierra already, you can expect

macOS security updates for at least a year or two. Even after

that, Apple typically still keeps updating many apps, like

Safari, compatible for even longer.

So, unless you really absolutely have to nave the new features

in Mojave, what you got will still be fine for some time and I

wouldn't worry about it until it dies (if it ever does).

Our 2011 iMac is running fine with OS 10.13 and I plan to keep it is service for several years. I have an older Mac G3 that maxed at OS 10.2.8. It earned me a significant amount of income long after any hope of upgrades had past. enough. You'll be fine.

As Allan stated, sorry it's not going to happen. If you want Mojave you will need a 2012 or later iMac. Your 2011 simply is not upgradeable.

FWIW, It will run High Sierra which isn't that different from Mojave so there's no reason it shouldn't be usable/suitable for a few years unless the hardware fails.

They are still fine Mac OS machines and yes, they make good Windows boxes too.