Pamela,
It might not be immediately obvious to your students that the most drastic changes in both tunes and texts occur when particularly creative and relatively sophisticated redactors become involved. Obvious examples are broadside rewriters, ballad editors, ballad chaunters, composers like Vaughan Williams, Elgar, Britten, Butterworth.

Many folksong scholars are not always aware of the extent that these sophisticated people can take a folk item, seriously alter it and then that item can come back into oral tradition.