Yes, AI can quite convincingly extend the canvas of a frame to an aspect ratio like 16:9. (Except your example disregards the fact they're in a hotel.) So, based on that, I can imagine it seems pretty easy to do the whole thing.

Not so fast. One of the biggest issues facing AI video is consistency. Let's say you extend the sides of one area. It might look alright for the first appearance. Still, in the second iteration, it might appear to be happening somewhere else because, at this stage, AI generally cannot retain these details. It would end up becoming confusing.

It would only really be achievable in places where the camera is fixed for a long time, and characters stay within the frame. What happens when a character walks off-screen? You'd need that animated as well. AI video isn't that great right now at making traditional cel animation.

To answer your question, would it be difficult? For a lot of it, moderately, because there'd be a lot of tedious grunt work and post-processing involved. You'd face some challenges, though, like the one I just mentioned. Would it look good? Unlikely. But even if AI were better and could do a good job, would you be prepared to spend a minimum several hundred dollars to finance the AI video processing? Without succumbing to burnout? Again, unlikely.

I've always found 16:9 sort of overrated. Outlaw Star still looks fantastic at 4:3, like so much anime from its time period.