For the purposes of proving that a document exists no, you really couldn't go back in time, if this were to happen it would be a monumental time for bitcoin, an

For the purposes of proving that a document exists no, you really couldn't go back in time, if this were to happen it would be a monumental time for bitcoin, and the old blockchain would be well known & referenced. So for the purposes of proving that you wrote that document it really doesn't matter if that were to happen.