Whenever any municipality having a population of more than four hundred fifty thousand inhabitants and less than six hundred thousand inhabitants, according to the last federal decennial census, has established a pension plan for the pensioning of the salaried members, and the widows and minor children of deceased members, of an organized fire department, no changes shall be made in the present and future benefits payable to such members or the widows and minor children of deceased members, except changes required to establish and maintain actuarial soundness of such pension plan as determined by the actuaries for the system and the members, unless an itemized list of the proposed changes and the reasons therefor is sent by mail to the designated representative of the members at least thirty days before the proposed effective date of such proposed changes and said municipality or its designated representative or representatives shall meet, confer, and discuss such proposed changes with the designated representative of the members.