I don't think most AAA games are scale-able at all. The game engines are built for Xbox One as the min spec. Developers will most likely build from scratch if they do it at all.
Possibly, depends on the engine being used. If its a AAA Unreal 4 game, probably a lot easier to scale down than say the engine Assassins Creed runs on. Psyonix is doing custom work for Rocket League because Switch doesn't natively support Unreal 3. Ironic, Wii U didn't support the newest engines causing issues, and now Switch doesn't support the older engine causing some issues. Luckily Psyonix has reported they are ahead of schedule, and things are going smoothly with the port.
Even when a port will take significant work, it still isn't truly from scratch. Treyarch's Wii team grew to about 40 people for the COD ports to Wii. Its not like the base material changes. I am pretty certain that the models and textures can scale down pretty easily with modern tools. Using lower quality shadows and lighting, or outright faking them with static texture maps is always an option. I am in the camp that Switch is far easier to port to than Wii was, and Wii saw a tremendous amount of ports and some did end up being ground up efforts. One of the best efforts from a third party was Prince of Persia Forgotten Sands on Wii. It was not a port, and instead a ground up title. It was excellent. Looked very nice (for Wii), and ran at 60fps.