One idea that just popped in my brain for people who want the prune faced look of the Emperor to be his true face (rather than resulting from Mace's reflection of Force Lightning which makes very little sense taking into account the lack of warping on Luke after getting the full blast of the Emperor's lightning in ROTJ).

As an alternative to blacking out the Sidious' face (as Magnoliafan and JasonN have done) it may be possible to add the prune face look onto those scene's (using similar techniques to those used by Ady for his redux of the ESB hologram).

In AOTC Palpatine begins to look gray faced and older but he looks younger in ROTS before his final transformation (presumably because Lucas had changed his mind about having a more gradual transformation).

If the full on pruny look was applied to Sidious it would be as effective a mask as blacking him out and having Palpatine lose the mask could be more instant (as attempted in a number of recent ROTS edit projects) and the audience would instantly link that face with those earlier scenes in the saga.