Do I actually need to do anything to go from GeForce to Radeon?
My GTX-1080 is getting a little long in the tooth, I'm thinking of going all AMD on my Linux Mint gaming rig here, but...is there anything I need to do or install or uninstall to switch to an AMD card from an Nvidia one?
I've never done this before on a Linux system; I've got my Intel/Radeon laptop, and my Ryzen/GeForce desktop and that's most of my Linux experience.
Aside from removing /etc/X11/xorg.conf because I had generated one via Nvidia's XServer settings - nope! The custom config there did prevent X from loading properly, switched to a tty to delete the config, restarted, and was perfectly fine afterwards.
Yes. X11 these days usually auto-configures on its own (to my understanding, at least) - when you generate one with Nvidia's settings it will add some stuff that is specific to the Nvidia driver, and thus once the card/drivers isn't present, then X11 can't start.
I had removed the drivers before swapping out the card in preparation, so I'm not 100% sure if said proprietary extensions doesn't load because of the lack of drivers, or the lack of the card itself - probably both to be honest.
But either way, X11 wasn't affected by the removal of the custom config, and there wasn't ever one present until I made one via nvidia-settings (other than, it started working of course).
Also, unless you're very dependant on some specific X11 apps, you don't need Xorg any more so I reckon you should switch to Wayland for a better experience (smoother, no screen tearing, high refresh rates, better multi-screen / multi-DPI handling etc).