Story missions will have Story and Veteran difficulty at launch, and you’ll be able to replay any chapters from Knights of the Fallen Empire or Knights of the Eternal Throne. Veteran difficulty is balanced for mid-tier endgame gear and a reasonably advanced companion and is meant to provide a significant challenge for the solo player. You won’t be locked into completing the content at that difficulty if it proves too hard, though, as players can scale the difficulty down mid-run if needed; scaling it up, however, requires a restart. You can catch the full presentation just below or read through Dulfy’s summary of the content therein.
Source: Official Forums, Dulfy