In no case may the lowering of the age of eligibility for a retirement pension as a result of section 10 of chapter 12 of the statutes of 1983, or the payment of such a pension have the effect of reducing a pension or other benefit granted to a person under 65 years of age by a collective agreement within the meaning of the Labour Code (chapter C-27) concluded before 20 June 1983 or by an arbitration award standing in lieu thereof and rendered before that date (1983, c. 54, s. 113).