On the question of a Sharp changing lyrics, the case of Elias Lonnrot (sp?) springs to mind.

EL collected many songs first hand from traditional singers and he used them to create the Kalevala. He justified his changes to the texts in that as he now knew the all songs he felt himself as good a singer of traditional rune songs as those he'd collected from. Therefore, changing the lyrics was in keeping with what they themselves did.

Lonnrot, of course, openly admitted his changes of lyrics and acknowledged his sources.