But that's the whole gist of the "planned" and "not planned" issue at hand.
Nomura does plan indeed ahead, but only one game after the other. While working on KH 2, he started to plan for BBS. When working on BBS (originally planned for the PS2!), somewhere then Days and Coded came in between because of Nintendo and Disney requesting them. While still working on the "handheld-trifecta", he began to look into bringing them together as preparation for KH III, which in the end eventually evolved into DDD.
So if someone asks about the entire story being planned from the beginning, the answer is clearly no.
Nomura is clever enough to leave many open ends in the games though, and as many people as there might be who scream "retcon" at every second corner, the Kingdom Hearts series itself has in actuality pretty few true retcons because in order to retcon something there would have to be something definitive to be established first.
To go back on topic though, if we would assume a preplanned storyline from the very beginning, the whole experience would be way easier to follow and comprehend and there would be many less potholes people would jolt over.
Yet, even if not everything was preplanned, the current way of doing things could also be in much better shape if not nearly 40% of important facts would be left to supplementary material rather than the actual games themselves.