(a) Any alteration hereafter made to an existing place of public assembly which increases its capacity or affects the status of its exits is subject to the requirements as set forth herein for places hereafter constructed.

(b) Where any requirement of this Part is based upon the capacity of the place of public assembly, such capacity shall be figured from the conditions which exist or are designed to exist, after alteration, conversion or change in type of occupancy.

(c) No provision of this Part as amended effective July 1, 1957, requires or is intended to require any structural alterations in or to a building existing prior to July 1, 1957, which complies structurally with the provisions of this Part or with variations granted by the board in effect prior thereto, except as may be needed to carry out the provisions of subdivision (a) of this section. Such a building constitutes one constructed in accordance with this Part.