With the Business Add-in ME_CHECK_ALL_ITEMS, customers are supposed to be given the option in purchase orders to reprocess all purchase order items whenever data in the header changes. At least, that is the theory.
According to my own tests, however, this only works with standard fields in the purchase order header, meaning the fields provided out-of-the-box by SAP (e.g., Incoterms).
It does not work with custom fields that were implemented in the purchase order header data on a customer-specific dynpro using enhancement MM06E005. So much for theory and practice.
Based on internet research, considerable effort and rather unconventional measures are required to trigger the reprocessing of purchase order items when custom fields in the header data are changed. Incidentally, this would also trigger implementations of the Business Add-in ME_PROCESS_PO_CUST (PROCESS_ITEM) once again.