I have recently changed to Debit Mastercard® but would want a refund for a payment made with my old Visa card, what should I do?

You are still eligible to claim refund for any purchase you made with your old Visa card, in line with the merchant’s refund policy, using your Mastercard.

If the merchant says that they cannot refund to a different card and ask for the card matching the part numbers on the receipt. You should explain that this is a replacement card you have received from the bank and that it is for the same bank account