It shouldn't be too difficult, the CPU is a little weaker than PS4's, there is less ram (3.2GB vs 5 or 6GB) and the GPU processing power is ~1/3rd the PS4's but otherwise is more feature rich than any other console on the market in terms of GPU. It is also an Nvidia Gameworks game on PC, meaning it will have optimizations for Maxwell/Pascal architecture found in the Switch.
The obvious first answer to reductions is in draw distance: You'd have basic objects at any real distance, think of how Zelda does this, it would be similar. Remove shadows and lighting from the scene at reasonable distance. What is important about the draw distance is that lowering it enough in combination with other reductions like foilage, lighting, shadows, textures and post processing, is that you reduce RAM usage, CPU draw calls, which could very well make up the difference of ~50% more CPU power on PS4 vs Switch, and would reduce GPU processing power. Obviously you'd have to lower resolution and the Gameworks optimizations would help the gap here too. It certainly would require noticeable reductions, but it's not impossible to see the Switch get the full version of the game, and for Square Enix, I think it absolutely makes sense to port over the full version of the game as the Switch is quickly climbing the Japanese market and by the games actual release, it will likely be the most prominent system in Japan and worth a double dip since it would be a portable version with current DLC included (like the PC version)
