https://boards.theforce.net/threads/marjorie-eaton-the-original-emperor.50033425/
This explains it a little bit. Someone in the thread assumes - and I agree - that maybe it wasn’t planned that the Emperor would have such a big role in ROTJ since the 9-film saga was still being bandied about at this point.
I’m also probably in the minority in that I prefer the original. Clive Revill sounds like an evil Obi Wan - potentially someone who could lure you to the Dark Side with gentle (yet sinister) persuasion rather than just goading you into permanently losing your temper.