118. The commutations of taxes granted prior to the date when this act applies to a municipal or school corporation, whether in the form of a reduction in assessment in relation to the market value of an immoveable, in the form of a reduction in the rate of taxes or in both forms at once, shall cease on 1 January 1972 as regards immoveables other than buildings and on 1 January 1975 as regards buildings, unless the period for which they were made expires earlier.