Whenever a contracting agency amends its contract to become subject to Section 21370 instead of Section 21369, a member who previously elected, pursuant to Section 21375, to remain subject to Section 21366, or a member who entered employment under Section 21366 after attaining age 30 and continued to be subject to that section, shall elect in writing within 90 days of notification by the board whether to be subject to Section 21366 or Section 21370.