A settlement between Visa, Mastercard, and merchants over a 20-year legal battle is crossing for the first time a red line for the credit-card industry by breaking the networks’ rule that forces a store that accepts one Visa credit card to accept all Visa credit cards. No longer would merchants have to “honor all cards,” instead they can reject credit cards that charge merchants bigger fees for each transaction, a concession Visa, Mastercard and banks have balked at for ages.