This part of Armchair's DD this morning (added to ibox, will be sticky note after MODs decide which one to give up, only allowed 4), but it addresses your question:
Q. How will open contracts be impacted?
A. DLA will evaluate open contracts on a case-by-case basis to determine whether a modification will be initiated to incorporate the new requirement. Open contracts judged to have the potential for significant future product deliveries or risk are likely to be modified, or possibly canceled in order to issue a new solicitation with the DNA marking requirement.
Q. How will existing inventories be impacted (e.g., at the manufacturers, authorized distributors)?
A. DNA marking is not intended to validate existing inventory, or existing product in the field. DLA uses other tools and processes to meet this need. DNA marking, in conjunction with trusted suppliers and approved DLA vetting processes, will apply to all new receipts of FSC 5962 in our depots.