California Code, Probate Code - PROB § 7060

(a) In addition to any other ground provided by law for disqualification of a judge, a judge is disqualified from acting in proceedings under this code concerning the administration of the decedent's estate, except to order the transfer of a proceeding as provided in Article 3 (commencing with Section 7070), if any of the following circumstances exist:

(b) A judge who participates in any manner in the drafting or execution of a will, including acting as a witness to the will, is disqualified from acting in any proceeding prior to and including the admission of the will to probate or in any proceeding involving its validity or interpretation.

(c) The amendments made to former Section 303 by Section 27 of Chapter 923 of the Statutes of 1987 do not apply in any proceeding commenced prior to July 1, 1988.