I'd imagine they'll have to significantly rework the lighting on some of these scenes. At Epcot, many of the scenes make heavy use of black lights to illuminate characters and the environment. Whereas these same scenes at Hong Kong use normal non-UV lighting fixtures. So I have to wonder if they're going to repaint any of the sets with non-UV paint as well.
On this matter, it should be noted that this swap-out won't magically make this a good version of the ride. Epcot's is still going to be the worst one out of all of them.
Aside from the superior AA's and lighting changes, HK benefits from having generally much more spacious and detailed sets. There are a number of places that showcase this. The lift hill (and the lengthier bit leading up to it) is a good example. Epcot's lift is extremely narrow with the walls very close to the boats. Much of this area was left dark and unthemed as a result, with only projections of snow effects being used. The ice palace up top is a tiny flat looking model. HK's lift was given a far wider space, they used this to add significant amounts of physical rockwork on the sides in front of the otherwise flat walls. The ice palace model is also much larger and more detailed, much more 3-dimensional.
The scene with Anna, Kristoff and Sven on the upper level is another major improvement.
Aside from the aforementioned differences in lighting, HK's set is much larger. Epcot's is extremely narrow and flat, with no illusion of depth whatsoever. Just figures in a mostly empty looking room standing right up against against a flat wall with little detail. Especially glaring because you approach this scene from an angle instead of head-on. The ice railing behind them is also embedded into the wall backdrop and isn't spaced out as its own layer. And the AA's are so close to the background that they actually cast shadows on the sky portion and cause it to appear even flatter as a result. The sky itself is just a blank void with snowflake projections added on top.
At Hong Kong, there is a significant amount of extra space. The AA's aren't as close to the flume, and they added additional props in front of them to improve the depth and help frame them. Helps disguise the flat emptiness of the floor they're standing on too (the lighting fixtures for the AA's are probably hidden inside these props btw). They're also spaced further from the background. There was enough room to make the ice railing a completely separate layered element from the sky, you can even see the sky through the spaces of the railing. It makes a big differences (especially in motion) and looks like a genuine balcony looking out onto the outside. The sky itself is also much more detailed, having painted mountains instead of just being a blank void of nothing. This entire scene now looks like a properly layered set with convincing 3-dimensional depth, the sky actually looks like it's far away and not just a flat wall shoved right up against the characters. Someone also had the sense to place some crate props behind Kristoff so it catches his shadow better and prevents it from casting onto the background and throwing off the illusion.
Incidentally, while the above comparison doesn't show this, the left side wall of the flume was actually themed a little at HK. It was left completely unthemed at Epcot. Your eyes are drawn to the right of course, but you're still going to see it in your peripheral vision at least, especially when you shift to looking forward. It's nothing impressive, but at least a little bit of extra effort was made to decorate that side to make it less distractingly ugly.
There's little they could do about some of the above issues. You can't add extra space where there isn't any. At most they could potentially paint more detail on the walls and fill out the left side better. But that would only go so far. In the end, Epcot's is always going to be the worst version of this ride no matter how many small improvements they might make to it.
