Cal. Code of Civil Procedure § 2034.610

(a) On motion of any party who has engaged in a timely exchange of expert witness information, the court may grant leave to do either or both of the following: (1) Augment that party’s expert witness list and declaration by adding the name and address of any expert witness whom that party has subsequently retained. (2) Amend that party’s expert witness declaration with respect to the general substance of the testimony that an expert previously designated is expected to give. (b) A motion under subdivision (a) shall be made at a sufficient time in advance of the time limit for the completion of discovery under Chapter 8 (commencing with Section 2024.010) to permit the deposition of any expert to whom the motion relates to be taken within that time limit. Under exceptional circumstances, the court may permit the motion to be made at a later time. (c) The motion shall be accompanied by a meet and confer declaration under Section 2016.040.