My personal dislike is when people intentionally write songs to well-known tunes..if they are so obscure that new words are a way to save them then I don't generally mind...

New words deserve a new tune, and there are plenty of people who could make one for you if only the words come to you. I just always hear the original words, which often are better, to the tune, regardless of what someone else has done.

There are cases where songs have just jumped onto tunes,and that is a different kettle of fish.

I think and hope there would be some legal issues if the new song became famous. And I think quite often it would irritate the original composter of the tune. mg