I'm sure it has happened, though I don't know of many examples. I know I heard that Angie Schworer asked if THE PRODUCERS would lower the key for Ulla's song for her. Angie could sing it in the original key (written for Cady Huffman), but she would sound better singing it slightly lower and she ended up getting the lower key. (And I think Angie played Ulla for a large portion of the Broadway run.)
