Article II, Section 1, Clause 5:
“No Person except a natural born Citizen, or a Citizen of the United States, at the time of the Adoption of this Constitution, shall be eligible to the Office of President; neither shall any person be eligible to that Office who shall not have attained to the Age of thirty five Years, and been fourteen Years a Resident within the United States.”
Some have repeatedly attempted to derail eligibility concerns by citing the conflicted national affiliations of the earliest presidents, Jefferson for example. But this is a dead end for that line of argument, because the phrase above,
“or a Citizen of the United States, at the time of the Adoption of this Constitution,”
clearly sets forth an exception to the NBC requirement, out of necessity. Where would the first generation of Americans be able to get their elder statesmen if they excluded Jefferson et al. In later generations, the more stringent standard would apply, because time would have produced a new generation of leadership fully capable of meeting that higher standard.
In any discussion of law, you start with the text, not the history. History may be informative, and even necessary to consider. But statutory language gets first dibs at creating meaning. Here you have two classes of citizenship, clearly spelled out by the text. There is no reason for that distinction if folks like Jefferson and others could qualify under the NBC criteria. It is inescapable then that merely being a “Citizen of the United States,” per the grandfather clause, is not the same as being an NBC. From there you probably have to explore what that difference might be mostly in external sources, history, decisions of the court, etc. But it is a given that there is a difference (as well as a secondary inference that NBC was somehow more difficult to meet), and a full accounting of that difference and its inferences must be made to get to the truth.
Hence the “grandfather clause” problem. You simply haven’t knocked around enough of these eligibility discussions if you haven’t encountered it before. I’m not saying it’s decisive. Only that it cannot be ignored in the formulation of any solution that might be worthy of being considered complete.
Then here is how I would answer your problem:
The grandfather clause is, I guess “or a Citizen of the United States, at the time of the Adoption of this Constitution”. That means that a NATURALIZED citizen, having probably fought for the new country during the Revolution, could become President.
There was an example, although I don’t remember the name. He came to the US in the 1760s, fought in the Revolution, and then was for a time Speaker of the House. As a naturalized citizen, he would ordinarily been prohibited from being in line to take the Presidency, but he was a citizen at the time the Constitution was adopted.
It also allowed anyone born outside the USA, who was a naturalized citizen in 1787, to become President. And again, I think there were two motivations for that. Politically, it made a lot of folks figure THEY could become President, and thus they voted for it. And second, most of those would have fought in the Revolution and thereby proven both their understanding of the new country, and their devotion to it. Those who helped deliver the new country shouldn’t have been barred from the Presidency due to being naturalized citizens, and they were not.