A health maintenance organization that provides pharmaceutical benefits shall notify all pharmacies under contract with the health maintenance organization in writing of changes in the pharmaceutical benefit program rules or requirements at least 30 days before the change is effective. Changes that require 30 days’ advance written notice under subsection (a) of this section are: Exclusion of coverage for classes of drugs as specified by contract; Changes in prior or preauthorization procedures; and Selection of new prescription claims processors. A health maintenance organization that fails to provide advance notice as required 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.