Seems to me that we're all agreed that these ballads survive (or sometimes, don't) as the result of innumerable choices made by their singers and listeners over centuries :-
* preserve absolutely as received - even if it no longer makes as much sense as it should in
   places
* add in some extra verses to clarify (taken from another, similar song)
* 'simplify' the tune to suit an aging voice
* lose the sense of the tune when trying to produce a seldom-sung ballad, so put it to a
   familiar melody
* hear a great version from another singer and take it on board - in whole or in part
* cut down the length, perhaps as a result of failing memory or perhaps to suit an audience

and so on ad infinitum...

But I'm curious to know why SoP is unwilling to change words when he seems quite at ease with the notion of changing or importing tunes. What's the difference?