That's already been explained. Yen Sid reset them to those forms while they're in the Realm of Sleep so they can relearn how to wield the Keyblade "the proper way." Now, as we've seen from these trailers it's not gonna change that much, but I think RoL combat will be crazier after this.
Granted, they're still using their self-taught methods, but by the end of the game, they'll probably find the 'proper way' of wielding their Keyblades to become Masters.
One question, though: Riku and Sora ARE getting more special moves to use in their command decks, right? Like ATV got stuff like Trinity Limit, Ice Barrage, Meteor, etc.
Because I noticed the deck commands used in the JF trailer were all the same as the demo and trailer shown at TGS...And that includes Riku and Sora being as far as the Tron Legacy world. They will get a wider variety as they move on...? That's the only thing I'm worried about in this game.
More like he was utilizing the co-op techniques with his Dream Eaters. While Sora teams up with his DE (as we've seen him dancing with Neko Cat and riding on Wonder Nyan), Riku uses his indirectly like power-ups.
As it turns out, the Dark Claw-thing was only the result of him fusing with Bat-Bat, alone. And he entered Firestorm by fusing with Goat-Horn.
It's when he fused with BOTH did he enter Blade Charge.
And to think they're supposed to team-up with FIFTY of these guys. The possibilites leave me so excited for this game.
