A source close to LucasFilm claims that there is a scene in Rogue One where Darth Vader has his limbs interchanged and repaired. The moment is meant to clear up some confusion as to why Darth isn't very good at swinging a lightsaber in his climactic duel with Obi-Wan Kenobi at the end of A New Hope. Especially since the Sith Lord is a master swordsman in Star Wars Rebels, which takes place only a short time before Rogue One.