Code Civ. Proc., § 2034.620

The court shall grant leave to augment or amend an expert witness list or declaration only if all of the following conditions are satisfied:

(a) The court has taken into account the extent to which the opposing party has relied on the list of expert witnesses.

(b) The court has determined that any party opposing the motion will not be prejudiced in maintaining that party’s action or defense on the merits.

(c) The court has determined either of the following:

(1) The moving party would not in the exercise of reasonable diligence have determined to call that expert witness or have decided to offer the different or additional testimony of that expert witness.

(2) The moving party failed to determine to call that expert witness, or to offer the different or additional testimony of that expert witness as a result of mistake, inadvertence, surprise, or excusable neglect, and the moving party has done both of the following:

(A) Sought leave to augment or amend promptly after deciding to call the expert witness or to offer the different or additional testimony.

(B) Promptly thereafter served a copy of the proposed expert witness information concerning the expert or the testimony described in Section 2034.260 on all other parties who have appeared in the action.

(d) Leave to augment or amend is conditioned on the moving party making the expert available immediately for a deposition under Article 3 (commencing with Section 2034.410), and on any other terms as may be just, including, but not limited to, leave to any party opposing the motion to designate additional expert witnesses or to elicit additional opinions from those previously designated, a continuance of the trial for a reasonable period of time, and the awarding of costs and litigation expenses to any party opposing the motion.