"The controversy over who shot first, Greedo or Han Solo, in Episode IV, what I did was try to clean up the confusion, but obviously it upset people because they wanted Solo [who seemed to be the one who shot first in the original] to be a cold-blooded killer, but he actually isn't. It had been done in all close-ups and it was confusing about who did what to whom. I put a little wider shot in there that made it clear that Greedo is the one who shot first, but everyone wanted to think that Han shot first, because they wanted to think that he actually just gunned him down.
"It's the same thing with Yoda. We tried to do Yoda in CGI in Episode I, but we just couldn't get it done in time. We couldn't get the technology to work, so we had to use the puppet, but the puppet really wasn't as good as the CGI. So when we did the re-issue, we had to put the CGI back in, which was what it was meant to be.
"If you look at Blade Runner, it's been cut 16 ways from Sunday and there are all kinds of different versions of it. Star Wars, there's basically one version—it just keeps getting improved a little bit as we move forward. ... All art is technology and it improves every year. Whether it's on the stage or in music or in painting, there are technological answers that happen, and because movies are so technological, the advances become more obvious. ... Changes are not unusual—I mean, most movies when they release them they make changes. But somehow, when I make the slightest change, everybody thinks it's the end of the world."
http://blastr.com/2012/02/george-lucas-on-han-shoot.php