1. A New Hope
The first change Lucas ever made to his films after release is one of the most important. When Star Wars first hit cinemas in 1977 it was called just that. There was no episode number, no subtitle, just the series' name. But George was very quick to change that. The name of the film had gone through various stages through production. The Journal Of The Whills evolved into The Star Wars, which ended up being the model for a six/nine/twelve part series, with Lucas spiritually making the fourth part. Smartly, he kept that number away from the title as people would be understandably confused by there being no previous three films. Once the film was a hit, however, Lucas gained the freedom to go back to his original idea, naming Star Wars 2 the Episode V we all know and, more importantly, rereleasing the original film as the more syllabled Star Wars Episode IV: A New Hope in 1981. This change proved instrumental in crafting audience perceptions of not only the trilogy, but the wider concept of Star Wars as a multi-episode story. But more importantly it showed incredibly early on that to Lucas his films were an ongoing project he was happy to continue tinkering with.
Well there you go - the Special Editions weren't all that bad (still not great though). Are there any more changes you approve of? Or do you still think the films should have remained untampered?