Not to poo-poo this, but this will be another stomping on the cannon of Ep IV. In that, Vader says "when we last met, I was but the learner. Now I am the Master." That was established in Ep III, when Kenobi beats him at Mustafar. Now they are going to have another epic duel to which Vader later acknowledges he was but the learner? He is a Sith lord.
I'm sure it will be a cool visual and all, but one of the few remaining lines of Kenobi or Vader talking about the past in the original trilogy that won't make sense anymore.