There's the rewriting that you do when you're putting a song together. But there is also a subsequent process in which the song keeps on changing.

Sometimes that can get blocked when a particular recorded version gets seen as the final version. You even get people protesting when a singer insists on letting those changes happen - for example Bob Dylan gets knocked for singing songs differently from the way he sang them 20 years ago. Not so much because the changes are for the worse (which in some cases they may well be), but because they are changes to a quasi-sacred text.

And the same thing happens with traditional songs - particular variants, sung by a singer on a particular occasion, will be treated as definitive, rather than as a moment in the songs life. And I'm not talking about where people set out to rewrite songs, I mean where the changes happen through mishearing and misremembering, and trying to reconstruct a forgotten line or stanza - the natural stuff that has always happened.

I was reading something Sydney Carter wrote about this, because it was germane to something that came came up in another thread. I suppose that's what set me to start this thread:

Having made a song, you sing it, but to write it is another matter. Particularly if, like me, you seldom sing it the same way twice; fresh possibilities will keep appearing. You change a word, you bend a note,; did it work, or didn't it? What you put down in the end is nothing but a variant...There is nothing final in the songs I write, not even the words, the rhythm and the melody. This is not an oversight; I would like them to keep growing, like a tree.