Any municipality may, by ordinance, accept the provisions of this part, as to its policemen or firemen, or both, but such acceptance shall not repeal, amend or replace, or affect the continuance of, any pension system established in such municipality by or under the authority of any special act or charter provision and all such special acts or charter provisions shall remain in full force and effect until repealed or amended by the General Assembly or as provided by chapter 99. When such ordinance has been adopted by the legislative body of any municipality, a certified copy thereof shall be forwarded to the Retirement Commission by the clerk of such municipality. The effective date of participation shall be the first day of July at least ninety days subsequent to the receipt by the Retirement Commission of the certified copy of such ordinance or resolution. The Retirement Commission shall furnish to any municipality contemplating acceptance of this part, at the expense of such municipality, an estimate of the probable cost to such municipality of such acceptance. If any municipality accepts the provisions of this part as to its policemen or firemen, or both, and such municipality provides survivors’ benefits for such policemen or firemen, or both, under authority of any special act or charter provision, any such policeman or fireman of such municipality for whom such benefits are provided may elect to be covered under the provisions of this part, provided such election shall be made within ninety days from the date upon which the municipality accepts the provisions of this part for such policeman or fireman and such election shall be in writing and filed with the clerk of the municipality and with the Retirement Commission. Upon such election, the survivors’ benefits for such policeman or fireman provided under special act or charter provision shall cease, and only those benefits provided under the provisions of this part shall be available.