When I stated that more than likely, all the riders were getting re-vamped, what did people think I meant?
Koji Seto is playing himself, because he was just in the show, and was available to film. The kid is playing Kiva, because that episode obviously takes place in the past.
Each Rider stint will only involve around 3 episodes. And yes, the riders are alternate versions of themselves, so having different actors, while it may upset some people, doesn't conflict with the story they're trying to tell.