Retcon

Retroactive continuity – commonly contracted to the portmanteau word retcon – refers to deliberately changing previously established facts in a work of serial fiction. The change itself is referred to as a retcon, and the act of writing and publishing a retcon is called "retconning".

When George Lucas re-edited the original Star Wars trilogy, he made changes directly to the source material, rather than introduce new source material that contradicted the contents of previous material. However, the current series of Star Wars prequels, do qualify as "new source material", and many fans have pointed out instances which apparently retcon elements of the original trilogy.

Star Wars movies

 * A rumor says that Lucas didn't intend Luke and Leia being siblings at first, but this is a concept coming to him very lately. This was the reason why a passionate kiss between them, seen in the Empire Strikes Back trailer, was cut from the final movie.
 * Possibly the first, and certainly the most well-known, Star Wars retcon was in Return of the Jedi: Obi-Wan's explanation to Luke about Anakin's true fate.
 * Ever since Episode IV, the Old Republic was said to have been established 25,000 BBY; but when in Episode II, Palpatine said that the Republic was only 1000 years old, the Ruusan Reformation was retconned into existence.

Expanded Universe

 * Since Episode IV, the Republic was known to have fought the Clone Wars, and the EU added several other conflicts, such as the Great Sith War; but in Episode II, it was stated that "there hasn't been a full-scale war since the establishment of the Republic." The above-mentioned Ruusan Reformation also handled this discrepancy.
 * The Imperial capital was originally referred to by writers as Imperial Center; when Lucas adopted Timothy Zahn's name of Coruscant, the name Imperial Center was retconned to be what the Empire re-named Coruscant.
 * Likewise, the capital city of Coruscant was originally called Imperial City; when Coruscant was determined to be an ecumenopolis, Imperial City was retconned to be the name of the actual city as opposed to the planet (itself being the renamed Galactic City).
 * The name of the Republic's leader was for many years called the President of the Senate; when the prequels revealed the title to be the Supreme Chancellor, the term 'President of the Senate' was retconned to be one of the Chancellor's several official titles (some say that this was the title before the Reformation).
 * In the movies, Red Squadron appears twice, in A New Hope and Return of the Jedi. According to the Expanded Universe, the Squadron was dissolved sometime in between and replaced by the Rogue Squadron. The reappearance is explained by that Wedge Antilles renamed briefly the Rogues as Red Squadron, to honor the memory of the heroes of the Battle of Yavin.
 * In the Jedi Prince series, Kadann and the others were trying to steal power away from Ysanne Isard. Isard is never mentioned in the series-in fact, it's frequently implied there is no Imperial leader other than Trioculus-but this retcon was created to fit the series into the established timeline.
 * It was established in the EU that Raith Sienar gave Tarkin the designs for the Death Star, and that Bevel Lemelisk developed the Superlaser; after Episode II revealed that the Geonosians gave the plans to Tyranus/Sidious, it was retconned that the Geonosians developed the details based on outlines given to them by Sienar and Lemelisk.
 * The show "Droids" showed C-3PO and R2-D2 having several adventures in the time between the rise of the Empire and Episode IV; after the release of Episode III showed them to be in the employ of the same people they are with in Episode IV, it has been retconned that the two droids somehow were separated from their owners after Episode III, then returned to them sometime before Episode IV. Although, it should be noted that the canonicity of the entire show is in doubt.
 * While the A-wings are supposed to exist since a little before the Battle of Yavin, they appear in the Droids series, which happens between the two trilogies. The identical R-22 Spearhead was then 'invented' as a predecessor of the A-wing, in order to explain the appearance of such ships in the Droids era.
 * The title Darth was supposed to be invented in the period of the New Sith Wars until the game Knights of the Old Republic introduced Darth Revan and Darth Malak, who lived 2,000 years before the beginning of the aforementioned period. It is generally assumed that the title was forgotten sometime between, so the tradition was lost. The first Sith Lord to use the title Darth also constantly shifted into the past: first it was thought to be Darth Bane, then Darth Rivan, and eventually, revealed after KotOR was released, Darth Ruin.
 * The BBY system used in many sources to represent dates in the Star Wars timeline is itself a retcon. It originated as an out-of-universe dating system and, after becoming popular, was retconned into an in-universe system used by the New Republic.