Yes, I know you're saying, "but aren't some of them dead? I mean, the actors who played them?" And you're right. That's why they'll use old footage for some of them, silly.
This sort of a big deal because, up until now, the new "Doctor Who" series hasn't really explicitly dealt with the Doctor's past incarnations. Sure, there have been hints here and there. And Sarah Jane, the Brigadier and K9, etc., have all appeared. But this marks the first time flashbacks have been made to the original show.
From the UK Sun:
Show chief Russell T Davies said: “It’s nice for fans to see the old doctors. And it helps kids get their heads round the fact there have been lots of them.”
The ten, from William Hartnell in 1963 to the latest David Tennant, will appear in "The Next Doctor" alongside new foes, the Cybershades. They are hairy, mutant cousins of the evil Cybermen — but they’re not the only enemy the Doc faces.