That's where you miss the whole point. Congress has the power to amend the Constitution and thus modify the founders' original intent.
Leo's analysis attempts to reveal the founders' original intent and demonstrate that the SCOTUS has confirmed the founders' intent with a ruling. But Leo has always taken the position that if the SCOTUS were to rule that Obama is a NBC that he would accept their decision. (As would I.) Likewise, he believes it is right and proper that Congress can amend the Constitution's original intent.
I just don't see how this fits with your previous statement that "a law which grants citizenship is, in and of itself, an act of naturalizing a group of citizens." The way I see it, this would create a sort of Catch-22 problem where the very act of redefining "natural born citizen" makes those people it includes not natural born citizens.