law: 161. Period for which rent is not liable to modification.-
  Save as provided in section 163, when the rent of an occupancy, an exproprietary or a hereditary tenant has been commuted, abated or enhanced in accordance with the provisions of this Act, it shall not be liable to be commuted, abated or enhanced unless-
  (i) in case of abatement or enhancement of rent-
  (a) the revenue payable by the landlord has been revised; or
  (b) there has occurred a decrease or increase in the productive power or the area of the land held by the tenant, as provided in clauses (b) and (c) of sections 153 and 154; or
  (c) the sanctioned rates have been altered; and
  (ii) in case of commutation of rent, a period of three years, or such longer period as may have been extended under section 83, has elapsed since the date of the last commutation order.