Also:

Whether "book" singers sing a folk song exactly as written in the songbook, as contrasted with feeling free to modify a song if learned by listening to it, is moot.

Early on, I learned about the "folk process," and that singers make changes in songs and ballads, sometimes inadvertently (as Pete Seeger said, "Not because of their memory, but because of their 'forgettery.'"). So I felt free to alter a word or a line if it sang awkwardly, or for other reasons. But NOT indiscriminately. I had to have a good reason to do so, and make sure that I didn't change the meaning.

And I know other singers, both well-known and obscure, who do the same thing and for the same reasons.

Don Firth