Famitsu explained that Sora’s actions are similar to the actions during his Kingdom Hearts 2 form changes. But the reason this is that they want players to be able to experience dynamic form change like actions even as normal actions here. Some scenery in the trailer may be tough to control. Adjustments will be made to the wall jump detection to make it easier to control.
All of a sudden, the character changes between Sora and Riku will be automatic. They wanted to surprise people with this, with the intention of taking away the feeling of security players. A lot of enjoyment can be made with this.
Sora and Riku have different stories with different bosses. Both of them visit the same worlds but in different orders. And it is rare they are ever in the same place. Different action and control methods suit each character.
Nomura is leaking into idea of changing Sora and Riku’s clothing, because some people might think that KH3D is a remake. Also, he is considering having them appear in their current clothing at the beginning of the game, altering midway through it.