I was wondering, knowing that originally Jenna was supposed to be leaving after this episode, if the "old Clara" was supposed to be the real ending. If so, I would have liked it because it was very reminiscent of Clara's goodbye scene with Old Matt Smith on Trenzalore, right down to them opening the Christmas cracker (which I don't think I've ever seen in person...must not be an American custom).
Knowing how much Moffat likes to toy with the fans, I suspect he only threw that in there to mess with people (since people were speculating about her leaving pretty much all year long), and that Coleman had already decided to stay by then.
And from recent interviews it sounds like Moffat and Capaldi both wanted her to stay another season anyway (and that it wasn't a matter of her forcing herself back into the show or something), so I doubt it was as big as a deal to the production as people want to think.