I think the reason they are dressed up in their KH1 clothes may be because of the training they are going through for the Mark of Mastery. If anybody watched Dragon Ball Z, you may remember the hyperbolic time chamber. For those of you that don't know, this chamber allowed people to train inside for years and only a few hours or so would pass by on the outside.
In KH: 3D, Sora and Riku may be doing the same type of thing, except they are traveling through their dreams and training as they go along. This would also explain why they are able to do all of those flashy moves as seen in the
and why Riku has an acid trip and sees 50 Soras flying through the air. Training like this would allow them to progress much faster, as they don't really have enough time to set out for another adventure to train while Master Xehanort is getting ready to come back.
Not sure if this has been said yet, but those are just my thoughts. Kind of a noob...
