Long debated among Star Wars fans has already been: did Han Solo shoot first or did Gredo? According to the re-release of The New Hope some years ago, Gredo did. However, the cantina scene in the re-release was a revision of the original. In the original, we never see Gredo get a shot off before Han shoots him. In the revision, Gredo shoots first and misses, and then Han shoots him.
Gamespot recently posted about George Lucas's explanation of the revision during his interview with The Washington Post. Quote:
"Han Solo was going to marry Leia, and you look back and say, 'Should he be a cold-blooded killer?' Lucas questioned. "Because I was thinking mythologically--should he be a cowboy, should he be John Wayne? And I said, 'Yeah, he should be John Wayne.' And when you're John Wayne, you don't shoot people [first]--you let them have the first shot. It's a mythological reality that we hope our society pays attention to."
This is what we call revisionist history. Die hard fans (like me) hate revisionist history.
As the writer with a completed series in front of you, you can naturally see all of the places where you might have done something different in the first installment, had you known then what would happen in the last installment. However, this whole-world view is not license to re-write a crucial scene in the first installment, decades later, to suit the last installment.
The cantina scene between Gredo and Han Solo, including the fact that Han shot Gredo first, is absolutely integral to arc of Han Solo's character.
Let me remind anyone who needs the reminder that we are first introduced to Han Solo as a smuggler, a thief, and a scoundrel. Viewers are under no illusion that Han Solo is a "cowboy". Comparing him to John Wayne is an insult to John Wayne and all other cowboy archetypes of his time.
Han Solo is not interested in doing the right thing unless there is a benefit to him. He is only interested in fortune, fame, and staying alive. He knows that there is a bounty on his head. Being a denizen of a dark underground world of self-involved, money-grubbing scoundrels like himself, Han also knows that Greedo intends to shoot him to collect on the bounty. He kills Greedo to save his own hide. He later agrees to take Ben and Luke to find Princess Leia, for the benefits it will bring him (fame and fortune).
Once the princess is rescued and returned to the Rebel Alliance, like in all good heroes' journeys, Han Solo is forced to choose between continuing to be a self-involved criminal or staying to help the rebels. Because he likes Luke and Leia, and because he knows what the rebels are doing is right, he chooses to change his ways and to fight alongside the rebels.
This arc of change for Han Solo continues as the trilogy progresses, and it is so appealing to fans, because it plays to our innate belief in second chances and our desire to see bad guys like early Han Solo become heroic good guys like later Han Solo.
The revision of the scene with Gredo to one in which Gredo shoots first makes Han's killing of Gredo a simple act of self-defense, rather than a character-defining moment of calculated self-preservation. His hero's journey is rendered practically useless.
Not only did George Lucas do a disservice to Han Solo's character, he also managed to inflame most of the Star Wars fan base. So, the lesson here is once you write it and publish it, however a scene irks you later on in the series, just live with it.