Any person electing to participate in the optional retirement program shall be ineligible for membership in the Public Employees' Retirement System of Mississippi so long as he is employed in a position for which the optional retirement program is available. If an optional retirement program participant assumes a position in state service other than as an employee of a state institution of higher learning, he must at that time begin membership in the Public Employees' Retirement System of Mississippi in accordance with Article 3 of this chapter.