So, this just happened.
That's only Paul McGann, back as the Eighth Doctor!
It was widely speculated that he would be back to film his regeneration scene in the 50th anniversary, but he denied it.
I was a bit dismayed, a bit gutted, but [the older Doctors] are not in it. There are all kinds of rumours doing the rounds. One gets tired of trying to refute things on Twitter and the like, but – take it from me – I'm not in it.