The Appellate Division, First Dept., has issued a decision saying that it's OK to change experts during the trial, as long as your CPLR 3101(d) exchange put your opponent on fair notice of the substance of the expert's testimony, and the substituted expert conforms to what was promised when he testifies. In S & W Home Improvement Co. v. La Casita II H.D.F.C. , NY Slip Op 07332 (1st Dept. 2009) , the Court also relied on the inclusion within the expert exchange of notification that another representative from the same company as the named expert might be called.
This was a mechanic's lien case, but what the First Dept. did here would obviously apply to any case involving expert disclosure pursuant to CPLR 3101(d). Therefore, medical malpractice and personal injury cases could arguably be affected by this decision, and I think it is a dangerous one. It is nice that the new expert stayed within the boundaries of the noticed subject matter. It is also nice that the expert exchange at issue happened to mention that someone else might be called at trial (as many such documents note). But if you are the lawyer on the receiving end of this last-minute switch, you've been placed at a disadvantage. You do not have time to research the new witness by obtaining prior transcripts or educating yourself about his history as an expert witness. You are thus deprived of the opportunity to develop key impeachment material that may affect the outcome of the case. You are, in a word, prejudiced. And trials are surprising enough when you are prepared to the gills.