Now Carrie Fisher on the other hand - she doesn't sound like she used to, decades of age will wear on a voice like that. What's the solution there? Re-cast? Digital de-aging?
I don't think that's something that could be done digitally. Altering a voice just makes it sound less like a natural voice. You can try adjusting its pitch, but that sounds fake, and it wouldn't deal with roughness or weakness in a voice or a change in performance style. Perhaps, in theory, someone could devise a program to modify a voice to make it sound younger or older, the way they have computer programs to make faces look younger or older. But it would be very complex, and to my knowledge, nobody's developed such a method yet, probably because we're a more visually oriented species so we focus more on how people look than how they sound.
Recasting seems the best solution.