What is just as interesting is the claim that the first three Doctors will be represented in the special through "studio trickery" and all of the remaining Classic Doctors are "keen" to return.
Assuming the idea is to get all eleven Doctors on-screen together for the episode's climax (and taking the explanation for Peter Davison's age in "Time Crash" into account), what I imagine we'll see is the faces of One, Two and Three grafted onto CGI body doubles, but the figures are ghostly and wavering - the vocal clips distant, echoing and garbled.
Eleven: I can't hear them - I can't understand what they're trying to say...
Four: I can - I'm the closest one chronologically to them. You see, the time differential between you and them has become too great for them to appear in any other way. (sad smile) I'm afraid I'm not too far from that point myself.
The three could put out some intelligble lines later in the scene, but only by putting forth a concentrated effort - otherwise Four would translate for them.