AITA for refusing to honour a customer's full loyalty card from the previous owner?

I bought a café business 10 months ago. When I first took over, I said I’d honour vouchers and loyalty cards from the previous management for a limited time. By August 2024, nearly all the old vouchers had been redeemed, so I put up a post on Facebook and signs in the window and by the till, letting customers know that we’d stop accepting the old loyalty cards at the end of September. I also offered to transfer any stamps from the old cards onto our new loyalty cards.

Yesterday, a woman came in, ordered a coffee, and handed me a fully stamped loyalty card from the previous management. The stamps were the old ones (I had deliberately changed the stamp design when I took over, knowing the policy would change after six months).

I politely explained that we’d stopped accepting those cards months ago. She got visibly annoyed and said I should honour it because she’s a “loyal customer.” I was confused because I’ve never seen her before—and I know all my regulars. I explained our policy, apologised, and even offered her a new loyalty card with an extra stamp as a goodwill gesture.

She doubled down, saying, “It’s just one coffee,” and insisted I make an exception. I told her that wouldn’t be fair to other customers who followed the rules.

She left annoyed, and now I’m wondering if I was too rigid. AITA?

EDIT for clarification - I’d like to clarify something regarding the accusations that I’ve "broken contractual laws" by not accepting her loyalty card. The sales agreement for the business specifically covers the stock, fixtures, and lease. It also explicitly states that I am not liable for any contracts or debts from the previous owner.

This includes, but is not limited to, outstanding payments to suppliers, contractual obligations to customers or clients, and any programs such as vouchers, event bookings, or loyalty schemes. I hope this clears up any confusion about the situation.