Sometimes he did. Its usually "this is what i wanted it to be like but couldnt afford to do it", which is pretty much true the majority of the time. I dont remember him saying anything about Greedo, though the above statement could apply since i have heard him justify it thusly. Also the new Prequel tie-ins such as the Emperor and Hayden he says that he wanted to redo them to accomodate the prequel changes so the saga seems more consistent.