I don’t want to tell you to do something that is not going to work so maybe would be best for someone else to answer you on this. I think if it was me i would have removed all the drivers first then shutdown and removed the nvidia card and installed the new amd card. This is the way i would approach this on Windows also.
Edit: Things have changed recently with the newer installs using dracut. Not sure if you have a newer install with dracut & are using grub or systemd?
Edit2: The way i solved this for another user but they were using grub and an older install without dracut.
Edit: Not sure what cpu you have either so you may not need ucode.