Hands up all those who play the guitar part to their songs exactly the same as the recording, note for note, every time around? What, so few? Then why are variations in the words to fit your preferences not also permissible?

As I see it, there's three main categories of changes to lyrics: deliberate planned changes, spontaneous "riffing" on the text (extra words like and/but/so/well/yeah/hey injected "to taste"), and actual errors. Errors like Davey Graham's version of "Nobody's fault but mine" where he's misheard "If I don't read and my soul be lost" as "Fire in my soul oh Lord". Just make sure you know which category your changes fall into! ;-)

Graham.