The thing about Star Wars is that even though the prints are not in the best shape, there are a lot of surviving elements compared with other films at the time. I believe a lot of this is because Lucas was smart enough to save original audio tapes, uncomposited shots and props. But he didn't keep any prints of the original film in good condition. Makes sense.
If there's any truth to what Lucasfilm has said, it's that they don't have any good prints readily lying around that could be easily scanned. They would have to search the archives like they are doing for the deleted scenes and maybe use a combination of prints and interpositives. It may not have been possible to have started that in May 2006 and finished with enough time for a September release.
But it CAN be done.

Any idea on the Jabba scene. Was the new CGI pasted over the old CGI?