A subscriber loses a card in March, the bank issues a replacement, and the subscriber updates it everywhere they remember — which is to say, almost nowhere. In June the renewal runs against the old number and declines. Nothing about that failure is mysterious, and nothing about it is the customer's fault. The card on file simply stopped existing, and the subscription kept billing a ghost. This is the failure class that account updater services exist to erase, and it is worth understanding what they actually do, because the marketing version — "cards update themselves" — hides the mechanics that decide whether yours will.

The card networks run updater programs that let issuers push replacement details — new number, new expiry — to merchants who hold the old credential, through their processor.…