What the FF intended was that We the People have the ability to make laws through our elected representatives and to amend our Constitution if we believed it necessary to do so.
The FF intended that the Constitution be the supreme law of the land AND that anything not prohibited by it or directed to a particular federal authority be left to the People to decide. (10th A)
xzins position is that in the realm of definition of "natural born citizen," there is no need for constitutional amendment, because that term means whatever Congress enacts and implies by legislation, and this is the fashion and authority for definition that the founders intended.
Your point about doing that via elected representatives still attaches, but the threshold for making a change is lower for a statutory enactment than it is for a constitutional amendment.
FWIW, I disagree with xzins. I believe the founders intended the term "natural born citizen" to have independent fixed meaning - independent of statute, that is. "Rules for naturalization" play for people who are not natural born citizens, and Congress was expressly granted the power to open and close the gate of naturalization.