My own view– admittedly preliminary– is that “natural born citizen” is a legal term of art. It comes from English common law, but it is subject to common law evolution, and therefore it was not fixed for all time in 1788. The best reading of the Eligibility Clause is that “natural born citizen” refers to persons who automatically become citizens at the moment of their birth. But Congress has the power to change (prospectively) who becomes a citizen at birth. If so, then Congress can partially but not completely alter who is a natural born citizen for purposes of Article II.