California Code, Government Code - GOV § 20551

A person who becomes a local miscellaneous member of a contracting agency on or after the effective date of the agency's contract to be subject to this article shall be subject to the benefits provided in Section 21353 but may, during an open 120-day period in each calendar year, to be determined by the contracting agency, make an irrevocable election to be subject, for all future local miscellaneous service with the contracting agency on and after the first of the month following the filing of the election with the employer, to the benefits provided by Section 20552 or Section 20553. A current member may make an irrevocable election to be subject to the benefits provided for in this article for all future local miscellaneous service with the contracting agency during an annual open 120-day period to be determined by the contracting agency and that shall be effective on the first of the month following the filing of the election with the employer.

Notwithstanding any other provision of this article, a member who has elected to be subject to Section 20552 may, at any annual open period next following the attainment of 10 years of credited service with the contracting agency, irrevocably elect to be subject, prospectively only, to the benefits provided by Section 21353 or Section 20553. Any person who makes that election may never thereafter change that election.