A friend of mine send me this article from the newest "Score" which is a Dutch soundtrack magazine and they have an interview with John Williams and he confirmed that he was asked by George Lucas to rescore parts of the first Star Wars movie, for the rest it's mainly about his past work.
"I am thrilled to have the chance to go back to something I composed in 1977 and be able to make the entire saga continuous theme-wise." John Williams tells us. "Notably abscent in the first movie is the Imperial March. Being able to insert that theme in the first movie will create a much stronger emotional flow throughout the Saga. After the release of Episode III George will enhance the original trilogy with additional scenes and special effects, adding my adjustments to the score this will seamlesly branch the entire Star Wars Saga into one massive romantic space opera. I think something of this magnitude has never been done before."