The encoder configuration is saved in Profiles, not Scene Collections. In your current configuration, you configure x264. Click Profile->rename and rename your current profile to "encoding with x264". Then click Profile->Duplicate and get a duplicate of your current settings. Give it a name like "encoding with nvenc" and change the encoder to nvenc. Now you can switch between both profiles via menu. This way you are also able to switch between different streaming providers without entering a new streaming key each time, by the way.