Subject to paragraph 6 below, where, under any of the provisions of section 2(1) of the 1975 Act (which provides for the alteration of the valuation roll in certain circumstances), the assessor alters the net annual value and the rateable value of any lands and heritages which are part residential subjects, he shall apportion the new net annual value and the new rateable value as between the residential and the non-residential use of the subjects, and shall amend the apportionment note accordingly.