it should have been the same dude from RotJ all along.
If you REALLY want to get technical, it should have been the uncredited Old lady with the Monkey eyes superimposed over her own, & Clive Reville doing the voice all along, seeing as how THEY were the Emperor in TESB.
Also, regarding something mentioned in a much earlier post. Nobody has disputed the fact that from the time we first see C-3P0 in ANH until the latest book in the EU that he's never had a memory wipe. Memory wipes were VERY common practice in droid maintenance. R2 & 3P0 were exceptions to that rule because they never had memory wipes from ANH on. But, it's more than obvious to me (and hopefully others) that 3P0 DID have at least 1 memory wipe in between ROTS & ANH.
Sorry for all the posts. I just keep forgetting more & more stuff that I want to say.