Note, this is not to do with on-the-spot problem solving, which may necessitate altering things as they go. This is to do with Luca$h's creation of a six film saga one film at a time. Now, I can allow for the fact that, when he finished SW, he wasn't sure if he would be able to make any other films after it. But the PT? I don't think he had all three episodes written before he started TPM but we can be pretty certain that he knew all three would be made.
He has claimed in the past that he had his story planned in the 70s, before SW was made. On the basis of the films, he seems to have trouble remembering what happened in the plot a couple of years ago, let alone 30 years ago.