I'd say that some of them have.
The originals came out when cgi was very much in it's infancy. Hell, they were still inventing it as they went along! A lot of the effects he's added, were done because the technology NOW exists to allow him to do it as he wants. He was constrained when the movies were made and ultimately frustrated that he couldn't do everything he wanted.
The flipside of that though is that some of it is indeed unnecessary tinkering. The extra dialogue and changing of scenes (Han not firing first springs to mind) are just change for changes sake. Also Lucas does seem to think more is better: "why only have 100 droids when we can have 10000?"
AND, if you can, have a look at an old version of the movies, on VHS if available, they look really horrible, pre-restoration. I'm pretty sure space isn't a murky brown colour.