Holy shit, I thought we'd have to wait until Christmas to finally find out! But also: Noooo, I'm not ready to say goodbye to Peter Capaldi! Until that tweet I had hope that Thirteen would look a lot like Twelve*, so we could keep PC a bit longer. But I doubt they'd do such an event only to anticlimactically reveal that the new Doctor actor is the old one.
* It should be possible, because didn't Romana consciously influence her new look? And Twelve really doesn't want to change anymore, so maybe he could regenerate into a very similar look.