I don't believe that deliberate change in a situation where the tradition is dead (cat among pigeons maybe) is part of any folk process, but there is nothing wrong with changing words in old songs; traditional singers did it constantly and deliberately, Sam Larner, Walter Pardon, Joe Heaney... have all spoken about doing it, in Walter's case, at great length, so what we have been handed down is the changed article.
Tootler's posting is dean on the mark IMO
Jim Carroll