Being a sad twat I am, I just listed to Lucas' interview thing he done at Celebration 3 over at Hyperspace.
Someone there asked him something like "when did you all have the story's planned?" and GL's reply was that he really only had 4-6 planned. Well it was one big story at first.
He never really wanted to make the PT, and that it started at 4 to make it look like the Saturday matinee serial. Although we know that Episode 4 was never mentioned at fist....
Only until Jurassic park was made, he then realised the effects were up to standard to make his story...the rest you know.
Well something like that.
But yeah, he does tend to change the story with every interview …