Like MissDisney said, In KH2 FM+ the developers made him look higher quality, they didn't actually physically change any features of Roxas, also the images you have are of different qualities, which is why the seem to be different. Also the lighting is different
Thing to remember:
Besides the CG scenes in KH there are 2 main types of facial and body expression in cut scenes:
1) Slide type, the face is less detailed, and when the mouth moves it just is in one spot now and then another spot the next moment when they speak, there is no moving from one spot, it's there and then it's somewhere else. An example of this is when you talk to the people on the first day in twilight town, they just keep on talking, but this allows you to see their facial movements, also look at the mouths, the seem to just be pasted on, and there is no inside the mouth features. also the clothes are blurred and less detailed The KH2 scene in the firrst set of pics is done with this style, because he's not talking, the developers left him with his standard look to save space on the DVD
2) Detailed type, unlike the above the Detailed type has enhanced clothes, proper mouth movements, and also the in-the-mouth features is was talking about, which there are alot more of in KH2 FM+, you just got caught up in the hype that roxas in kh2 looks different to roxas in kh2fm+, so just get it in your head:
IT'S NO BIG DEAL, THEY JUST MADE HIM MORE DETAILED AND MORE OF HIS SCENES NOW HAVE THE DETAILED STYLE OF MOVEMENT AND CLOTHES, THAT IS ALL
There is no physical difference whatsoever, one image is simple, the other detailed, that's all thereis to it
