1If, in pursuance of a proposal made before 1st April 1964, or made on the ground of a change in the occupier or in the circumstances of occupation, the gross value shown for a hereditament in the valuation list is varied after 13th November 1963 then, as regards any rental periods (whether beginning before or after the variation) the 1963 gross value of a dwelling being or forming part of that hereditament shall be ascertained by reference to the gross value as so varied.