5 sentences. I'm assuming they all run on the same graphic engine, not stating they're exactly the same. And yes, all the assets are reused in KH games. Don't know what you're talking about.
The fact that they look similar doesn't mean they used the same engine to create them - unless you are talking about the HD cinematics or something like that which I guess might be the case.
How much was reused between Kingdom Hearts I and Kingdom Hearts II? As a director, what’s exciting about melding the two game universes?
Nomura: For the first question, although there are many similar elements, we actually recreated the game engine [for Kingdom Hearts II] from scratch, so we didn’t import any data from Kingdom Hearts I. It’s 100% new. Although it’s the same world, it’s completely new.
Kingdom Hearts II Interview (IGN) - Kingdom Hearts Ultimania
Even KH1 + 2 don't use the same engine, BBS uses the engine from Crisis Core etc. Obviously, with that many titles on different systems and different years of release, a variety of engines were used.
I do agree that UE4 look really freaking beautiful.
But correct me if I'm wrong, but would the development time increase even more, since they have to transfer all the work the have done up to this point from their current enige, to UE4?
I mean I'm happy that they made the dicision to switch engins to speed up the development, but wouldn't there only be more time added now they have to tranfer. Only wondering, that's all...
Depends on how much work they have done with Luminous. It will be worth it though if Luminous is a constant fight with the engine, which by the sound of it, it is.