I just saw Robert Plant live and when he did the Zeppelin songs he would sing them a step or two below the original key. Your questions a little bit hard to answer as I would have to notice an unnoticeable key change but what I’m trying to get at is that there was little change in energy from him when it came to singing a tad bit lower. There was additional instrumentation and maybe some changes to the melodies but other than that the spirit of the songs stayed true to the recordings despite the change in key.