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—
in case of abatement or enhancement of rent—
the revenue payable by the landlord has been revised; or
the sanctioned rates have been altered; and
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.