More comparisons…
https://www.youtube.com/watch?v=vlEG7awIuck
I always assumed it was done with tweaking, rather than re-recording the lines with JEJ or using different audio takes.
Honestly, I’ve never noticed the tweeking in the Special Editions.
Regardless, there’s still no doubt Vader’s voice in ‘Star Wars’ is different than his Empire and ROTJ voices; granted they each fit their respective movies.