At the risk of stating the obvious, that is what the folk process is all about: change, evolution, call it what you will. It is to be embraced and celebrated. Otherwise, our living tradition of folksong will become like the rather more dead early music tradition, in which the songs and tunes are treated more like museum pieces (as interesting as this may be).

I'm surprised no one's mentioned Newlyn Town, recorded inter alia by Martin Carthy and myself, or the Irish Newry Town.

All of these various versions evolved from an original (perhaps The Flash Lad). However, the original version of a traditional song does not mean it's inherently better, just older.