The original 1973 theatrical cut a.k.a. the studio cut is very similar to the 1978 re-release version. Lucas was able to restore a few scenes that had been taken out by the studio in its original release after his success with Star Wars and create his director's cut (the same thing happened with THX 1138 that same year);

The Louie, Louie sequence at the hop.
Terry's exchange at the used car lot just before the first hop sequence.
Bob Falfa singing Some Enchanted Evening from South Pacific after he picks up Laurie.

The audio was also remixed in Dolby Stereo for the reissue.

The original theatrical release has never been released on home video. Also, don't know if this info is true or not but according to IMDB:

"In the original release, John Milner is listed as having been killed by a drunk driver in June of 1964 in the closing segment just before the final credits. When the movie was reedited in 1978, the date of his death was changed to December of 1964, most likely in anticipation of the release of its sequel, More American Graffiti."