As a singer of mostly "traditional" songs, I'm of several minds on this age old question. By the way, being of "several minds" comes easier with age ... try it some time.

Here's where I allow myself to change a word in an old song:

1: the word is so obscure that no understanding would happen, but I try to take care of that problem in the song introduction.
2. The word is so blatently insulting ... but I usually just caution the audience befor I sing it.
3. I forget the real word, then I politely excuse MYSELF and pass the mistake off to "the folk process"!

CHEERS, bob(deckman)nelson