Pick ten of your favorite movies, then go through the trivia on IMDB for them all, and good odds at least 9 of them will mention an earlier script with many differences. This is really no surprise at all.
And contrary to popular belief, Lucas didn't have the whole saga planned. He just did each movie one at a time, fitting them in as he could. If he did have it planned, he would have had a set bible and there would have been much better consistency between the prequels and the sequels. There have been at least a couple of times he has denied ever planning Episodes 7 through 9. Before the prequels ever got put on paper.