Here's what I think. You can change the tune within eight consecutive bars of music to claim a copyright. Where you run into trouble is the lyric. You can't copyright a title. For example
James Taylor and George Harrison wrote "Something in the Way She Moves" (different lyric and tune).
Most Dylan, Guthrie, and others have antecedent tunes for their lyrics. For example the tune for Dylan's "Masters of War" is Jean Ritchie's "Nottamun Town". Dylan's "God On Our Side" was "Patriot Game" or the traditional "One Morning In May". Guthrie's "This Land" was taken from an old hymn and is close to the Carter family's "Little Pal O' Mine".
Simon 's "Scarborough Fair" has the first part which is traditional
and the second "Canticle" composed.
Folk songs were meant to be re-written slightly. This is why they
are folk because the tunes are somewhat recognizable.