If they resided here at time of Constitution ratification they became citizens like anyone else at that time....who qualified
Yes, by naturalization due to formation of the union. Same sort of effect occurs when a territory becomes a state, the citizens of the state become naturalized citizens of the union. In order to accommodate a "born citizen" clause, and to have a pool of eligible candidates for the office, the constitution also includes a clause that anyone a citizen at the founding (even those naturalized BY the founding) were eligible.