I did JOSEPH... and my Narrator couldn't sing most of the score the way it was written (The original Bill Hutton/Laurie Beechman key), so we had to drop most of the score down three steps (saving the opening "Any Dream Will Do", "Close Every Door", "Grovel, Grovel", "Who's the Thief?", and the "Any Dream... Finale"). I think when you take the key for JOSEPH down, it transforms the score to a very different mood, darker and almost contorted. The part of Joseph is written as a tenor because the character is young, light and buoyant, not a gruff and older man. I disagree with the changing of key in general, but especially when it affects the show itself. Andrew Lloyd Weber wrote in the keys he did intentionally.
And if she'll say, "My darling, I'm yours!" I'll throw away my striped tie and my best pressed tweed, all I really need is the girl...