Whats Different?

Because the A.U. is rewriting both the prequels and sequels, several major aspects established as cannon need to be reworked, here is a summery of some of the major changes already established:

The Republic:  The Republic is treated much more as a realistic and nuanced government, with the empire being an extreme tyrannous form of the Republic.  After the Empire is defeated at Endor, many of the same systems remain in place, only with the Emperor removed and democracy placed over top.  As the story progresses, the Republic slowly slips down the path of tyranny and by the later story has become practically indistinguishable from the Empire.  In the same way, the Old Republic is treated as a bloated and controlling state slowly falling into chaos until reactionary forces seize power and declare the Empire.  Obi wan may romanticize the Republic, especially in the face of the Empire, but it was far from perfect and many of its flaws lead directly to the Empire's creation.

Technology:  Because the Star Wars Universe is so ancient, it makes sense that Technology would be on a more static trajectory than is depicted in the prequels.  New destroyer designs and other massive weapons like ATAT's would be relatively static in that they have been around for some time and are unchallenged in battlefield supremacy.  Technology, especially military tech, would change mostly during big wars like the Clone Wars or the battle against the Rebellion.  It also makes sense that the New Republic as well as the old would use star destroyers and other tech similar to the Empire in the original trilogy.

Sith and the Dark Side:  In the Original trilogy (the movies not the scripts) the Dark Side of the force is conceived of as something nebulous and mysterious, not simply a mirror image of the light side as it has become in the Prequels and beyond.  Dark Sided Jedi are still Jedi, as there is no overarching label for them like "Sith" in the prequels ect.  This returns the basic dichotomy of Dark vs Light side to how it was in the Original Trilogy.  As well the Sith are now simply Jedi from another Empire, defeated long ago by the Old Republic.  They have survived as Nomads in hiding, continuing their teachings until one day they might emerge from hiding.  Sith, like Jedi, can be either Dark or Light sided.