Section 7. If a town votes that official ballots shall be used for the election of town officers, it shall at the same meeting determine what officers, in addition to those required to be elected by ballot, shall thereafter be chosen by official ballot, and determine the number and terms of office, if not already fixed. No change shall thereafter be made in the officers to be chosen by official ballot, or in the number or terms of office thereof, except at a meeting held at least sixty days before the annual meeting at which such change is to become operative.