The funny thing about swapping out Hayden for the scene is that basically Lucas is saying the guy who betrayed his friends, mentors and fellow Jedi, slaughtered a room full of younglings, killed hundreds or maybe thousands of jedi, force choke demoted a number of admirals, committed an untold number of war crimes and atrocities, and was either directly responsible or an accessory to the genocide of literally billions, gets to spend eternity as a dashing 20-year-old force ghost while Obi-Wan stays in his sixties. And no wonder Yoda looks pissed. He's probably thinking "WTF!!! This is my friggin reward!?! If I'd turned to the dark side centuries ago, I could have spent the last 800 years kicking ass, chasing tail and boozing it up, then do the ole' switchback on my deathbed and I'd have been a young ghost too. Crap!"
I also thought that instead of simply switching Vader to the James Earl Jones voice you would have thought a sound designer would have been able to come up with some kind of a blend between Christensen, JEJ and the breathing mask. That completely took me out of the scene, because you knew it was no longer the same person. I think it was either just plain lazy or they figured the audience would be too stupid to understand.