If the installed AMD Catalyst components, such as display driver, CCC, AMD HDMI audio driver, etc., are from a package that supports both gpus on the same OS environment, then it shouldn't be an issue really.
Despite this, i always highly recommend to fully uninstall the old drivers first, along with any leftover folders and registry entries (DDU in safe mode and ccleaner do the job nicely), before uninstalling the old adapter and installing the new adapter. You then install the latest drivers on a clean environment.
Better safe than sorry.
I won't comment on the nvidia card...