Using a combination of different VFX methods, I've used what I call "diegetic de-aging", to bring Ellen Ripley's appearance in 1986's Aliens, closer to that of her appearance in 1979's Alien. Between the two films, Ripley is in Cryosleep, and according to most-sci fi tenets, you don't age in Cryo. Or you age slower, much slower.
This was meant as a continuity experiment, hence the concept of "diegetic de-aging", in service of the character of Ripley, nothing else.
Check out the split-screen version where you can see a comparison:
Or the fullscreen version, which just showcases the diegetic de-aging version: