In my experience, it is easy,should the need arise,to make up new words to go with a well-known & popular tune, but it is not even half so easy to put a well-known song to new music !! Yes,it can be done, but the tune used would be one that was already in the memory -banks .In the Middle Ages,for example,when many of what we think of as "traditional" songs were,in fact, a medium for communicating & recording information about important events,the tunes or melodies used were often taken from the Church,from hymn tunes that were already in the national consciousness. A lot of early American "folk music" draws heavily,in my opinion, from a pool of Irish,Scottish,English,German,French & other continental music for its tunes ( and rhythms ).For a modern example ?? ---- I play a tune called "Varsoviana" which I refer to as being a traditional Polish dance -tune. But for many people , especially Martin Wyndham-Reed,it is very much the song "The Babes In The Wood ".!! At the end of the day ,I guess to most people,it doesn"t really matter,although I really do think that it will be the tune that"s remembered long after the words are forgotten or,as is so common,distorted.