It's worth bearing in mind that some trad songs were collected, or otherwise arrived in the 21st century, in a 'damaged' state. But that I mean that they no longer have the integrity and power that the original writer intended.

While this 'patina' has value - (the story OF the song may be as interesting or even more interesting than the story IN the song) we shouldn't forget that if a song is to be sung out (as opposed to merely residing on some dusty shelf in a book or recording) it needs to work as a song, and not just as an historical document.

The oral tradition was often as destructive as it was constructive, and we probably have the 'better' versions of some songs as much because of a bit of judicious repair along the way, by someone who understood how songs work, as by any accidental adaptation.

Obviously if we all radically rework the classics just for the sake of it then we're doing the tradition no service, but if a singer (as opposed to an archivist - though these may be the same person wearing different hats at different times) encounter a rhyme, line or even a whole verse which jars or is unnecessary, then - if they're planning to sing the song out - I think they do have a duty to try to fix it, while admitting the change and the source for anyone who wants to go back to the original, (or decide if the fix works or not)!

Anyone who resists or resents this is failing to take on board the reason that songs exist in the first place.

Also, I'd like to point out that most of the comments in this thread seem to refer to changes made to the well-known, 'good' trad songs.

Most of the major modern reworkings have been made from more obscure songs.

The truth is that a majority of trad songs are neither well-known nor good (I have a shelf full of books here to prove it). Anyone taking one of these (as I did with The Discharged Drummer and The Ballad of Long Preston Peggy, for example) and making a new work from it is committing no crime...

Though, as ever, it's up to the listener to decide if the new version an improvement or not.

On the whole, it's the versions that work best in performance that tend to carry the day

Tom