The Food and Drug Administration has specified in its chemistry guidance for pre-market submissions for food contact notification that "end-test" or compliance test extractions are "quality control test methods for verifying whether a particular product is equivalent to the material that served as the basis for the regulation" (click here to view a copy of the guidance). If you change suppliers, you would need to conduct the end-tests on the new plastic containers to be assured that they are equivalent. Generally speaking, the regulations require compliance with the end-tests of any article that is placed into commerce for which regulatory compliance is claimed. Therefore, it is the responsibility of the manufacturer/distributor to assure that each product meets the end-test.