"One of the most futile debates that many fans engage in is what is and what is not official Star Wars cannon/history. If you would carefully read what has been published in past issues of the Star Wars Insider, you would already know how George feels about Star Wars cannon . . .
In one of his interviews with the Insider, George stated that the reason he is making the prequels is so that he can drastically change our current perception of the classic trilogy. If he weren't going to change our perception of the trilogy, then he said he wouldn't even bother to make the prequels.
Certain "things" that will occur in the prequels will change the Star Wars cannon that we now take for granted in the classic trilogy. Of course, George has already changed our perception of the classic trilogy when he made changes to the films in the special edition. For instance, prior to the special editions, official Star Wars cannon stated that Han Solo shot and killed Greedo and Greedo never even got off a shot. However, with the special edition, cannon was changed. Greedo shoots first and Han shoots second.
What George is trying to tell you is that there is no such thing as rock-solid Star Wars cannon. He can do whatever he wants in the prequels and can change the classic trilogy in zillions of ways. What we now perceive to be official cannon from the trilogy will drastically change once the prequels are released. For instance, many assume that Kenobi and Vader (Anakin) entered a duel that ended with Vader falling into a molten lava pit. However, Kenobi told look in ANH that Vader betrayed and murdered Luke's father. Then in ROTJ, Kenobi is forced to admit that Vader is Luke's father. So what's going on here? What other convolutions of the truth will occur in the prequels? Will Obi-Wan continue to give his own spin or interpretation of the truth in the prequels?
Will George reveal to us that many of the truths we cling to concerning the trilogy will ultimately turn out to be lies. How many lines of dialogue that are spoken in the classic trilogy will turn out to be lies, half-truths or misperceptions of the truth?
How will what we see in the prequels affect our perception of the trilogy? At this point, only George knows. But one thing is certain. With the prequel trilogy, George has already warned you that some of the Star Wars cannon that we now take for granted and assume to be rock solid will be thrown out of the window. Some of the changes to official Star Wars cannon may "bug" you a little or in some cases a lot. Some of the changes will be ingenious while others may fall flat.
Once the prequels have come and gone in the theaters. We will finally get to see what George believes to be the real history of the Star Wars galaxy. It's going to be a very interesting ride." (Source: Anonymous Lucasfilm Informant)