I am well aware that my two paragraphs somewhat contradict each other. But at the same time i still feel that it changes hans character. Yes it was technically self defense in either case, but the fact that Lucas had to change it is like see, Greedo was actually threatining Han's life, it wasnt just all talk. which it well could have been. You basically have 3 versions of the scene, the OT, the SE, and the DVD version
OT - Han is 'threatened' by a bounty hunter, he's a rogue, he's used to dealing with these types of situations. He actually has the the upper hand and makes his move before Greedo even knows whats up. You can even argue that he reacted with more force than was necessary in the situation giving him that 'bad guy' character that is usually argued because he didnt know if Greedo really planned on killing him.
SE - Similar, we see that Han is already preparing for the worse, but now he's only responding to almost getting his head blown off and being damn lucky that he didnt. He Not only did he really not have the upper hand in the situation, he almost got killed by a pathetic bounty hunter that couldnt hit a target 3 feet in front of him. Now Han is just some guy that deals with unsavory people, but no one that is a real threat.
DVD - Again, Han is getting prepared to 'defend' himself, but he's still not fully in control of the situation, he's not as quick as he once was, now Greedo not only has time to get a shot off, he's almost as quick as Han. This may seem just as good as the OT version as first but remember Han is technically on the defensive, so he needs to prempt the strike in order to protect himself, not a response like it was in the SE. Greedo is probably more pathetic cause he should have been more ready to shot Han, not a rushed shot at the same time his prey fires at him, if it was Fett (or any other decent bounty hunter), you think Han would have even been able to draw his weapon let alone fire it. And again Han lucks out that his shot hit (though this can be seen as he's just more skilled and a better shot because he was able to hit is target under such a quick shot) but he still just barely gets his head blown off.
I like the OT Han personally, you get the impression that this has happened before, its not the first time he's been held at gunpoint and he's not worried because he's got it all under control. why else would he start taking out his blaster under the table before Greedo actually threatened his life. plus in the new versions he still preps his blaster, but now even with that prep he's not quick enough to off Greedo before Greedo gets a shot off.
So sorry if my previous post was unclear. Basically, imo, the reason it seems Lucas changed the scene is so its clear that Han was acting in self defense. But by doing so it changes Han's character because we dont get that confidence in Han's ability as a smuggler who deals with shady people and backstabbing on a daily basis. Hopefully this clears things up.
-Darth Simon