You'll have to perform a complete uninstall of the Nvidia Drivers. I would remove the drivers before swapping the card but I don't think it will cause any issues if you remove them after you swap. After you are 100% sure they are gone give it a good ol' reboot and verify, then install the new drivers for the Radeon. shouldn't cause you any issues unless the build from manufacturer has some other conflict. I have done a few swaps between Nvidia and AMD and there were no major issues unless I messed up something. But I built my own PC and had a clean windows install not a "ASUS" windows build if that makes sense. I don't see where there would be an issue myself. If anyone else knows something I don't then add your input.
I7-4790k OC 4.9 1.375v
MSI Gaming X GTX 1080 OC core:2055Mhz Mem:5504Mhz
-----------------------------------------------------------------------
If you like what I post, hit that Rep button.
-----------------------------------------------------------------------
If i'm posting here it's only because I'm at work and have nothing better to do other than solve your problems.