The UE engine is generally easy to work with. But Square needs to optimise the Switch version a hell of a lot more than the PS4 version. That will take quite a bit of time it seems. They don't have a code they can just copy paste on Switch. If they had the PS4 version would have ran at 1080p/60fps. I mean they obviously could not optimise the Switch port, but it would look and run pretty bad if they did just that.
"He mentioned that for those who would consider purchasing the English version, the version of the engine used is different, so he considers it “closer to a remaster than to an update.” Unfortunately implementing those changes on the Japanese version would require a lot of time and is not realistic in terms of costs".
Did you actualy wrote link that I put? Thats a point, if one version of game is opimaised bad it dont mean same will be for other versions for other platfomrs or even for same platform for difrent market (like case is with PS4 West release) link that I provided proving that Western release will be more remaster compared to Japanese release, because of different engine (updated UE4 engine) and better optimisation, they most likly doing Switch version now on that new engine also.
I read what they changed on the PS4 version and guess what they are not changing either the resolution or the framerate. Otherwise Square would have shouted that of the roofs. Sure they changed the menu's, English voice over, difficulty, dashing and the camera.
The only reason they used a new engine is because it was cheaper. The PS4 version is probably still not optimised and why would they, it will sell short of 500k in the West anyway. Also the updated engine was probably easier for porting the PS4 version to Steam.
The Switch version will obviously have much better optimisation because it needs to. That's why it will take a lot longer than porting the PS4 version. Hell the Switch might even be build from the ground up. It's also the version that will likely outsell the PS4 version, so obviously there is way more budget for optimisation. Unfortunately that comes with time and that's why Square is delaying this version.
So I truly doubt if Square Enix is going to do much outside of what they announced for the PS4 version. They will make an effort for the Switch. Because they need to and because it's the version that will outsell the PS4 and PC version combined. Besides the term remaster usually doesn't include much.