The metamorphosis of a ca.1795 ballad about "pills and salts of white mercury" into The Bad Girl's Lament" and "The Streets of Laredo" is one of the best documented evolutions in Engliah ballad scholarship. ("St. James Infirmary Blues" is a stretch, however.)People, ballad printers included, evidently loved this stuff.

The late Kenneth Goldstein edited an entire Folkways album around 1960 showing examples of the sequential alterations.

I believe Philips Barry first pieced the evolution together in the 1930s. For a specific reference, see N. Howard Thorp, "Songs of the Cowboys," ed. by Fife & Fife (N.Y.: Clarkson Potter, 1966).

No, the book ain't on line.