A municipality may withdraw its acceptance of the provisions of this part, for either its policemen or firemen or both, by the same procedure as provided in § 7-323b for acceptance of this part, provided, if survivors’ benefits are, at the time of withdrawal, being paid or due to be paid to survivors of policemen or firemen, or both, of the withdrawing municipality, the withdrawing municipality shall be liable to the fund for any additional sums necessary to continue payment of the survivors’ benefits throughout the period when any such survivor shall be entitled to benefits under the terms of this part.