McDiarmid replaced the original actor cast for the Emperor on Return of the Jedi, and no it was not Clive Revill.

I don’t think they ever revealed the actors name but he was older than Ian and couldn’t wear the special contacts in his eyes. So it was serendipity that Ian ended up playing Emperor Palpatine at all.

As for the new shot in Empire since 2004 i hate it. It does not match his look in ROTJ, and the dialog is much worse than the original.

Also i can understand Lucas decision to put Hayden into Return of the Jedi and that it makes his 1-6 edit make more sense, but i never liked that change. Maybe i’m blinded by nostalgia, but i always liked Sebastian Shaw in that scene.
I also really hate the new voice for Boba Fett. I like the actor Temuera but his lines are delivered all wrong. Like he was recorded over the telephone reading a phone book or something. No gravitas or menace to his intonation.