CCP §1258.270: (a) A party who is required to exchange lists of expert witnesses and statements of valuation data shall diligently give notice to the parties upon whom his list and statements were served if, after s

(a) A party who is required to exchange lists of expert witnesses and statements of valuation data shall diligently give notice to the parties upon whom his list and statements were served if, after service of his list and statements, he: (1) Determines to call an expert witness not included in his list of expert witnesses to testify on direct examination during his case in chief; (2) Determines to have a witness called by him testify on direct examination during his case in chief to any opinion or data required to be listed in the statement of valuation data for that witness but which was not so listed; or (3) Discovers any data required to be listed in a statement of valuation data but which was not so listed. (b) The notice required by subdivision (a) shall include the information specified in Sections 1258.240 and 1258.260 and shall be in writing; but such notice is not required to be in writing if it is given after the commencement of the trial.