Writersoul, if the original song for the music was one full of profanity then it is inherently tamei. I don’t think it would be obvious to someone listening who has never heard the original but the fact remains that just listening to a tune that was written for impure lyrics, even if Jewish words have been put to it, will damage – whether the listener realises it or not. We once had a class in sem where our teacher (wife of a well-known rav) taught us that music goes into our souls, not just our ears. I agree with oomis: because those operas you talk about were written mostly by religious, G-d-fearing Christians who had a sense of morals (although not all of them did), whereas some Jewish music comes from contemporary non-Jewish music written by completely immoral people. And whether or not the words have been changed, the melody will still make it’s impression.
I have a challenge, for all those who know Phantom of the Opera: try to put Jewish words to Point of No Return, and see if it sounds any more kosher (have to confess that I do like POTO, though).