It was possible to edit Ian's lines to match Revill's performance ... but they had to be sped up a huge amount. Someone with professional Lucas equipment could do it better.
If Ian is ever supposed to be in Empire, the scene should be edited to match the original picture, perhaps with only the sound being changed. Lucas didn't do this, he just ruined the scene.
Revill's performance is of course much better ..... Ironically, on the film I'm restoring now, The Thief and the Cobbler, in the "butchered version" Clive Revill wound up revoicing the original actor, Sir Anthony Quayle! So he's been on both sides of that fence. Amusing. I'm using mostly Quayle lines with a couple of Revill ones snuck in.