A contracting agency whose contract is effective on and after January 1, 1974, that does not become an employer for purposes of Chapter 9 (commencing with Section 20790), or a contracting agency that ceases to be an employer, shall be subject to all provisions of the retirement law as it exists on the date of contract or on the date a contracting agency ceases to be an employer, whichever the case may be, and as it may be amended thereafter excepting the provisions of Chapter 9, other than Section 20834, and those amendments thereafter as are expressly made inapplicable to a contracting agency until the agency elects to be subject thereto.