I don't think that the fact I wrote a song gives me any ownership rights over it in terms of how it's sung.

Songs have a life of their own and change; what is special about the music we talk about as folk music is that it recognises this in a way that other types of music don't the same with stories.

Sometimes people make changes that are changes that make a song worse rather than better. The person who wrote a song has the same right as anyone else to think, or in extreme cases even say "you are wrecking that song" - but no more than anyone else. Whether their opinion is given value depends on other things.