The city council of any Class 1 municipality may grant pension plan benefit increases to retired employee participants, and their beneficiaries, in terminated or inactive former pension plans of any Class 1 municipality, provided the increases shall not exceed the increases granted to retired participants in the City of Birmingham Retirement and Relief System created under Act No. 929, 1951 Regular Session, as amended. The benefit increases may be retroactive but shall be granted only after the city council receives the opinion of a licensed actuary regarding the cost of any increase.