Brendan Moody wrote:
(I can't find any information about Gerald Campion's part.) Additionally, impressionist Paul Jones replaces Tom Baker, who was unwilling to participate. Otherwise, I think the major cast was intact.
As he was only on location and the study set, all of Gerald's scenes (and the Policeman's) were recorded in 1979.
Christopher Neame recorded his lines from LA.
Some of their voices have changed a little, but since 90% of Skagra and his ship's voices weren't recorded it just means the for 10% they sound younger, rather than the rest sounds older. Indeed the one scene recorded that did feature Shirley Dixon as the voice of the ship was her last scene, and the Doctor had clearly been tinkering with the programming and personality by then anyway.
I don't think Daniel, Victoria or Lalla sound any different.
Had it been done for more stringent purposes, you could perhaps redub the 70s footage of the deceased actors (with K9 thats rather easy) But Ian Levine was very defiant on keeping as much of the original as possible. Well all of it.