Yes, sometimes I change the words. *ducks flying tomatoes*
Whether or not this is a mortal sin depends on a lot of things:
- Whether the song originated in the oral tradition (vs. having a known composer)
- The number of known historical versions of the song, and the reliability of each version (a lot of collectors edited shamelessly, especially in the 1700s and 1800s)
- The extent of the edits (did I just tweak a few things so it’s in modern American English instead of having some Scots? did I change a few lines but leave the content alone? did I change the plot?)
- Whether I take the trouble to study multiple versions of the song, so I know exactly what I’m doing
- Whether you think this violates “impersonality on the part of the performer”, an important aspect of traditional ballad singing style
- How much of a purist you are
I usually won’t acknowledge really small changes (e.g. changing “came” to “is come” so that it rhymes better), especially when the song exists in enough similar-but-not-exactly-the-same versions that the ‘correct’ word to use in a particular place is sort of undefined. Any larger edits will be noted.
Why do I change the words? So that I don’t have to sing Scots words like “gae” or “maun”, which would sound odd in the middle of what could otherwise be a modern American English line (and believe me, if I tried to affect a Scottish accent it would be even worse). To make it rhyme better (read up on the Great Vowel Shift if you’re curious why Shakespeare used so many half-rhymes). To incorporate elements that I like from different versions — usually this means adding some small inconsequential plot element or bit of description.
I have never gone so far as to make an original change to a song’s plot (although I can’t guarantee I will never do so). When there’s an alteration begging to be made, it seems that usually someone else has been there before me, so it becomes a problem of choosing or mixing versions. For example, I like to change two lines of Tam Lin to remove the implication that one character rapes another (because if you leave the rape in, it ends up being a rather creepy rape-culture story, and the rest of it is so beautiful that it doesn’t deserve that fate).