I think that too much is made out of taking a trad tune and reworking it in a new song. There are very few folk tunes rewritten or otherwise that are original. Now if you talk about Cole Porter, Irving Berlin, Frank Loesser or George Gershwin et. al., then that might be a different story although even the creative George based "It Ain't Necessarilly So" on a liturgical Jewish piece perhaps unwittingly. Another interesting use is "Nottamun Town" (Jean Ritchie) and Dylan's "Masters of War". I remember the story of i think it was Maurice Jarre who was surprised to find that he wrote a song to a Russian national anthem.
Stravinsky said something to the effect that composers don't plagarize, they steal.
I think that Paul Simon added to Scarborough Fair with the Canticle and made it a slightly different content.
Sometimes a little thing can make a melody have a different dimension either through another lyric or an addition in the arrangement.
Frank Profitt got the royalties and sued the Kingston Trio for "Tom Dooley" but Frank Profitt did not write "Tom Dooley" and the K.T. copped their arrangement from the Folksay Trio on Asch/Stinson with Erik Darling, Bob Carey and Roger Sprung. The K.T. got their famous "hiccup" in the song from the Folksay Trio and I believe it was that "hiccup" that made the song famous.
So the idea as to who did what with what melody is much ado about nothing.....
unless of course you put some new lyrics to "Night and Day" by Cole Porter or tunes by Jerome Kern or Kurt Weill. Then I think you'll hear from their publishers.
Folk songs endure because they are ...familiar... and this means that the melodies have been heard before in other songs whether they are rewritten or not.