Credit for prior service shall be granted to each local member who rendered service to a public agency if that agency or a function of that agency is assumed by a contracting agency or a public agency that thereafter becomes a contracting agency.

The future annual costs incurred pursuant to this section shall be determined in the manner prescribed for actuarial investigations and valuations in Article 1 (commencing with Section 20460) of Chapter 5 and shall be made public at a public meeting at least two weeks prior to the election by a public agency to be subject to this section.

This section shall not apply to any contracting agency nor to the employees of any contracting agency until the agency elects to be subject to this section by contract or by amendment to its contract made in the manner prescribed for approval of contracts.