Now I don't know why Lucas continues to fuck with his films. Honestly, I don't. If I had to make a guess, I'd say it was something psychological, some form of OCD. Or maybe it's just because he can. Whatever the reason, it doesn't matter. I'd even argue that Lucas' "intentions" don't matter. I've heard people argue that they're his movies, so what he says goes. But that's not entirely true, in my opinion. When an artist creates art, he can tweak it and fix it and reinterpret it as much as he wants privately, but once he puts it out there, it belongs--at least in part--to the public. If Leonardo was alive today, we wouldn't let him and his paintbrush back into the Louvre because he suddenly realized the Mona Lisa should have had bigger tits.
Authorial intent certainly means something, but it doesn't mean everything. If I write a short story, and you get something out of it besides what I intended you to, then good for you. It doesn't mean you're wrong, and it doesn't mean I've failed in writing my story. Lucas claims we shouldn't see Han as a "cold-blooded killer," but that just shows how distant he's become from these characters, because I'm betting that no one ever saw Han as that. After all, Greedo had his pistol pointed at Han, and had just told him that he was going to enjoy killing him! That's not cold-blooded; that's self-defense. Or look at it this way--if we see that scene as Han being "cold-blooded," what are we to make of Han swooping in out of nowhere during the final battle to destroy the TIE fighter that never fired a shot at him? Isn't that the equivalent of shooting someone in the back? Now, of course he was defending his friend, so we don't fault him, just like we don't fault him when he shot Greedo. First.
One last thought: even if you believe that Lucas has the right to "clarify" his original intentions for the character, it still doesn't change things. Lucas apparently forgot that he had a script. That's the thing about having stuff written down--it makes it pretty easy to see what those original intentions were. This is from the script:
GREEDO: Going somewhere, Solo?
HAN: Yes, Greedo. As a matter of fact, I was just going to see your boss. Tell Jabba that I've got his money.
Han sits down and the alien sits across from him holding the gun on him.
GREEDO: It's too late. You should have paid him when you had the chance. Jabba's put a price on your head, so large that every bounty hunter in the galaxy will be looking for you. I'm lucky I found you first.
HAN: Yeah, but this time I got the money.
GREEDO: If you give it to me, I might forget I found you.
HAN: I don't have it with me. Tell Jabba...
GREEDO: Jabba's through with you. He has no time for smugglers who drop their shipments at the first sign of an Imperial cruiser.
HAN: Even I get boarded sometimes. Do you think I had a choice?
Han Solo slowly reaches for his gun under the table.
GREEDO: You can tell that to Jabba. He may only take your ship.
HAN: Over my dead body.
GREEDO: That's the idea. I've been looking forward to killing you for a long time.
HAN: Yes, I'll bet you have.
Suddenly the slimy alien disappears in a blinding flash of light. Han pulls his smoking gun from beneath the table as the other patron look on in bemused amazement. Han gets up and starts out of the cantina, flipping the bartender some coins as he leaves.
HAN: Sorry about the mess.
See the part where Greedo shoots? Neither do I.