As far as I am aware Bert was circumspect about what he altered or put together.

But since folk songs are subject to change all the time - does it really matter? All sorts of people do it, not just Bert. That's what some folk singers do.

What would have mattered was if he had copywrited it. But he didn't. If you want to alter it before singing it, go back to what you might think is an original, make up a new verse or an ending or a beginning, re-write the story or turn it upside down, go ahead. We might get a great new song out of it.