I would highly recommend uninstalling AMD catalyst control. It is simple and is the proper way to do it. I once switched between two Amd cards without uninstalling the drivers and it screwed a lot of stuff up. Such as my boot times and Ethernet connection, I had to run internet wirelessly for a while. I am not saying that this would happen going from amd to nvidia but I would be on the safe side. It takes ten minutes at most and it will save you a headache.
As far as the new card goes, after uninstalling the drivers from the old card. Just disconnect power from the Amd card, and remove it from the PCIE slot. Then drop the new GPU in and connect power. When you first boot with the new card install the new drivers for that card. It should have come with a disk with all the drivers on there. After that check online for the most up to date drivers. If there isn't any then you are good. If there are then download and install them, and your good to go.