First when it was broadcast by The BBC shortly after its original run (they didn't carry the other two series), and edited by them for timing. Most of the cuts are just nips and tucks to long bits of dialogue, but there's a particularly egregious one in Episode 13, not only losing the scene where Motti conspires with Tarkin to overthrow the Emperor, but violating the show's format whereby major scene transitions are always accompanied by music.
Second when NPR decided to lengthen the closing credits. The cut material includes a rather touching scene in Episode 2, where Leia tells her father about a pleasant stroll she took on Alderaan. Unfortunately most of the home audio releases use the cut version.
The Other Darrin: Most of the movie cast was unavailable, though they did get Mark Hamill and Billy Dee Williams to reprise their respective roles in the first two plays. Due to the thirteen-year gap between TESB and ROTJ, both roles ended up recast for the ROTJ play. Anthony Daniels does reprise the role of C-3PO, as he does in just about every other canon Star Wars adaptation, and likewise R2-D2 still sounds the same.
Luke Skywalker:A New Hope and The Empire Strikes Back featured Mark Hamill reprising his role as Luke. However, he declined to return for Return of the Jedi, and Joshua Fardon was cast in his place.
Lando Calrissian:The Empire Strikes Back featured Billy Dee Williams reprising his film role, but Lando was played in Return of the Jedi by Arye Gross.
Boba Fett: In The Empire Strikes Back he was played by Alan Rosenburg, but Ed Begley, Jr. assumed the part in Return of the Jedi.
Wedge Antilles: This character was heard in all three series, and was played by three different actors.
The Emperor: Unlike in the films, however (at least, before the Special Editions) the Emperor was played by the same performer (Paul Hecht) in both The Empire Strikes Back and Return of the Jedi.