It's not really about fixing when it's not something that's broken.
For almost all video games, there are different models for characters, I think everyone basically guesses this, but there are reasons why they do it. Having a close-up high poly model and one for scenes where it's not as important are something that they do in order to focus on what needs to be focused on.
Sora and Riku already have new models to begin with, in fact most of the game I'd wager has new textures. But, going through the time to build and rig a 3D face for a low-poly model definitely wouldn't be a priority for them when it's not as important. For scenes like the kids in BBS Sora, Riku, AND Kairi were given high poly models because they were shown, close up, for incredibly important scenes. But it wasn't just a final mix thing, it was something they did for the international releases - and the wait between those releases was unbearable.
Are they noticeable for 3d now? Yeah, but I think it was a choice they made on purpose. Though, yeah, you don't have to like it.