What happens when an expert moves from testifying to consulting in a case?

It’s fairly common for a valuation analyst to sign on to a legal case in a “consulting” capacity. Not only might this reduce costs during the preliminary phases of litigation, it shields communications between the expert and attorney—and any preliminary reports or work papers—from disclosure and discovery, absent “exceptional circumstances” (the standard under the federal rules). Should the case heat up and head for trial, the attorney can “promote” the expert to a testifying expert, subjecting his/her opinion to the applicable rules, including the recent amendments in federal court and Tax Court that now protect most draft reports and communications from discovery.

But under what circumstances may an attorney change the use of an expert from testifying to consulting?  For instance, an attorney may seek to de-designate its expert to prevent the other side from discovering an opinion/report that is unfavorable to the client’s position. According to a new article from the Gibbons law firm, at least one court has held that once an expert is designated as testifying, she or he "can be deposed and possibly called at trial,” particularly when that expert has already issued a report. By contrast, another cited case followed the majority approach that requires "exceptional circumstances" to depose a de-designated expert.