The best advice would seem to be "Make up your own version of the melody line (une which fits what you hear)" then teach that and if enough people learn it from you it will become the official one.
The same works with lyrics - when the Fureys sang Eric Bogle's "Green Fields of France" they got the words wrong (or changed some). Since then there are a large number of singers who only use the version they have heard by the Fureys, some would probably tell Eric Bogle that he sings it wrong!
Quack!
GtD