I saw fans on Reddit wanting Witwer to overdub Peter Serafinowicz’s work in TMP because Serafinowicz isn’t “part of the franchise.” These fans are dicks.
Of course, you can make up a canon reason for switching actors, say “Witwer is Maul’s voice after Maul lost his lower body to Kenobi.” As for McDiarmid redubbing the lines, I’m glad they got Witwer’s permission.