Cal. Rev. & Tax. Code § 18523

If a joint return is made under Section 18522, any election (other than the election to file a separate return) made by either spouse in his or her separate return for the taxable year with respect to the treatment of any income, deduction, or credit of the spouse shall not be changed in the making of the joint return where the election would have been irrevocable if the joint return had not been made.