I heard that she was just being shown the past in a vision or something like that, and Luna was probably talking to Celestia who was behind Twilight or something
yes, the animatic shown at Comic Con. at first people thought it was Nightmare Moon's return, but if you listen closely enough it's actually Twilight viewing the past from Celestia's point of view. so actually, it's witnessing Luna's transformation to Nightmare Moon. which is pretty damn interesting too. it doesn't make much sense for Luna to revert back, it kills a lot of what has already happened
it also has the production number of 401, meaning it's part one of the premiere.