.: Special Edition Changes
In 2006, tieing in with the release of the original theatrical versions on DVD, the official Star Wars site launched a feature called "what has changed", in which it attempted to catalog every change that was made in 1997 and 2004 to the films, using before and after pics. There were a few examples it missed and an error here and there, but otherwise it was an impressive and useful resource, which sadly was taken down (along with a lot of other useful content) when the site was re-designed and downgraded for the Clone Wars feature film in 2008.
Fortunately, originaltrilogy.com user Doubleofive had downloaded and saved them all.
List of changes to Empire Strikes Back
List of changed to Return of the Jedi
These only list the visual changes. By my counting, Star Wars alone has approximately 120 new or altered shots--commulatively, quite a different experience watching the film. In time, I will repost the image collection on this page. There, are, however, many audio alterations in the film, many taken from the various sound mixes of the past. The 1997 Special Edition was the first 5.1 channel mix Star Wars had. Most theatres played it in Dolby Digital, although there was a DTS mix made as well. The Dolby Digital mix was released on Laserdisc in 1997 and is currently the best 5.1 source for the film (see the editorial Can't Even Get the Special Edition Right). The sound was totally re-mixed in 2004 for the DVD release, where a few new additions and alterations were also made.
1997 Audio Changes:
Star Wars
-For an unexplained reason, Han's last line to Greedo ("Yes, I'll bet you have") has had the "Yes" edited out, though you can still see his mouth forming the word. This was restored for the 2004 mix.
-"Whizing" sound effects have been added to the asteroids that pass by when the Falcon comes out of hyperspace at Alderaan. These initially appeared in the 1993 mix.
-C-3P0 describes how to disarm the tractor beam while R2-D2 shows blueprints on the monitor. This line originally appeared in the mono mix of 1977, which was never released on home video.
-The extra sound effects added during the 1993 mix in the cellblock control room shootout (the "broken glass" sounds) are not present.
-As Han and Chewie are being chased through the Death Star, a stormtrooper yells "close the blast doors." This was originally in the 1977 mono mix as well.
-Computer sound effects are present when Luke and Han power up the gun-pods during the Falcon's escape from the Death Star. These were originally in the mono mix.
-Before Red Leader dies, the line "he's on your tail" was added.
Empire Strikes Back
-When Luke runs out of the wampa cave, the sound of his lightsaber deactivating is heard
-Before Rogue 2 is hit, Luke can now be heard saying "This is it!" (prior, you could still see him saying it but there was no audio)
-In the asteroid field, C-3P0 says "there's nowhere to go" after he says "This is suicide"
-"You're lucky you don't taste very good" was changed to an alternate take of "you were luck to get out of there". The original line was an improvisation by Hamill while the new line is what was scripted.
-While Luke is running with Yoda on his back, Yoda says "Yes, run"
-Extra traffic sound effects have been added to Cloud City (such as when the train car passes under the landing platform)
-The trooper who shoots C3P0 has been redubbed, and C3P0 says "hello" twice now.
-C3P0s complaining as they are led into the carbon freezing chamber ("...don't do anything foolish") has been mixed so as to be clearly audible (in the original mix, the chamber sound effects drowned him out).
-Luke grunts when Vader pushes him down the stairs
-Lando says "attention" twice over the P.A. during his announcement.
-Music has been restored after the Slave 1 flies away.
-Luke screams in his fall from the platform into the air duct during his Vader duel. Fans have surmised that this was the Emperor's scream from Return of the Jedi reused, although this has never been officially verified. The scream was removed again for the 2004 mix.
-Vader's "bring my shuttle" is replaced with an alternate "Alert my stardestroyer to prepare for my arrival." Lucasfilm claims that this line is, in fact, James Earl Jones and was originally recorded for Star Wars, but many refuse to believe this (aside from not sounding like him and not being in the script, it's unclear where in the film it would have fit).
Return of the Jedi
-Jabba's palace band has had their song "Lapti Nek" replaced by a new song called "Jedi Rocks" for the extended musical sequence.
-As Han is reaching to Lando in the Sarlac pit his line "it's alright, trust me" is replaced with an alternate "it's alright, I can see a lot better."
-The "Yub Nub" musical piece had been replaced with an alternate.
2004 Audio Changes:
Star Wars
-When R2 hits the switch to open the escape pod door, a beeping sound has been added.
-C-3P0 says "we've stopped" before he tells R2-D2 to wake up on the sandcrawler.
-Ben's "Krayyt dragon call" has been replaced with a higher-pitched wail which sounds very similar to the Boga from Revenge of the Sith. As the 2004 mix was made during early post-production on Episode III, it is probably an earlier version of that sound.
-Han's "I'll bet you have" is now the original "Yes, I'll bet you have."
-After the stormtroopers search the captured Falcon, one says "there's no one here" as they pause in the doorway before leaving.
-A sound effect has been added to the stormtrooper who hits his head on the door entering the control room.
-When Luke and Leia are trapped on the Death Star chasm, the echo effect to the dialogue is more naturalistic. The gunshot sound effect of a .44 magnum is also now a standard blaster.
-"He's on your tail" is removed.
-The music in the battle of Yavin sequence has had it's left and right surround channels accidentally flipped in many shots. In other shots, the music has been dialed down very low or made inaudible, such as the swell during the very first dive bomb. Lucasfilm has claimed this is a deliberate artistic choice.
Empire Strikes Back
-Jason Wingreen's iconic Boba Fett voice has been re-dubbed by Temurra Morrison, who played Jango Fett in Episode II.
-Luke igniting his lightsaber when he first approaches Vader now has the sound effect from Return of the Jedi mixed in.
-Luke's scream during his fall has been removed.
Return of the Jedi
-I am not aware of any strict audio changes for Return of the Jedi, other than in the new material. The most notable new audio is Jar Jar Binks exclaiming "Wesa free" during a new shot of Naboo at the end.