I'll come at this one from both sides - for now anyway.

For songs I have written I know that after a few weeks/months/years what I sing differs from the text what I wrote. That is an inevitable consequence of the way a singer makes a song comfortable for him/her to sing. Sometimes the change is for better because I might have thought of a better word that I used originally. Sometimes the 'better to sing' word may be less poetic - but the point of a song is the singing.

If other people want to sing my songs that's also what they're for and if they feel the need to change them I'm big enough not to cry. The only time I might feel the beginnings of a tear is when they change what I regarded as an important metaphor or other subtlety in the writing that actually changes the sense. Then I am likely to grieve for my poor molested baby.

The same, of course, applies to tunes. They change and, above all, the phrasing will change to fit the performance and the singers style. That, too, is what singers do, unless they are deliberately setting out to copy soemone elses performance (not my thing but probably very flattereing for the person being copied)

So, if you come from there with written songs it has also to apply to collected, traditional songs. If they have passed through an oral process then each of the successive singers will have made it 'comfortable' for her/himself. The collector will have mediated in writing the song down. A recorded performance can be on a good day or a bad day. But basically most of the best singers I know will take a song and make it work for them at the level of words, meaning, message, music and performance. A good,sensitive singer can't do 'wrong'

martin