In the Wikipedia, George Lucas gives his explanation for changing the cantina scene to having Greedo shoot first:
Star Wars creator George Lucas explained the change by stating that he wanted to make clear to children that Han had "no choice" but to shoot Greedo.
But there's also this photo of Lucas during the making of Indiana Jones and the Kingdom of the Crystal Skull:
I read elsewhere that Lucas wanted to be sure to keep the PG rating for Star Wars, to keep it more family friendly, not surprisingly, for financial reasons. Unfortunately I can't find my source for that any longer. The reasoning given above could very well be translated to what I think I read about the MPAA rating being the reason for the change.
Is there any documentation or background showing any additional reasoning from Lucas for this change?