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.
While I find this concept intriguing (and it's basically the way Tony Lee's The Forgotten
ends), I have a difficult time seeing it as viable from a production standpoint.
No, it's not the CGI.
It's that this gives a half-dozen actors incredible leverage over the production. If you want the fourth Doctor, you have to get Tom Baker, after all. "Two lines of dialogue? Essentially a cameo? Pass." The negotiations necessary to get all the actors onboard and satisfied with their roles would make Middle-East peace look achievable by comparison.