When Star Wars: The Old Republic’s upcoming expansion, Knights of the Eternal Throne, drops in a few weeks, players will be able to replay through any story missions in order to experience any differences in the narrative.
For the moment, players can choose to replay the missions in either story difficulty, or veteran difficulty. A third level, master, will be added at a later date. Veteran difficulty pits even more challenges at the players, and allows them to earn additional command XP. Master will, presumably, have even more rewards.
All of this information comes from a recent developer stream.