At least 30 days before the change is effective, an administrator of a plan that provides pharmaceutical benefits shall notify in writing all pharmacies under contract with the plan of any of the following changes in the pharmaceutical benefit program rules or requirements: exclusion of coverage for classes of drugs as specified by contract; changes in prior or preauthorization procedures; or selection of new prescription claims processors. An administrator that fails to provide advance notice under subsection (a) of this section shall honor and pay in full any claim under the program rules or requirements that existed before the change for 30 days after the postmarked date of the notice.