It is not a problem to refund a customer through Collins if they have replaced their bank card.
If a customer makes a payment through Collins and you need to refund it to them, but they have since replaced the bank card that they used to make the original payment, the refund will nevertheless be returned to the bank account linked to that card.
If the customer has closed their account completely:
If they paid with a debit card and closed their account through a bank switch, the refund will go to the new account.
If they paid with a credit card, or a debit card and completely closed the account (without switching) the money will go into a suspended account, and they will need to contact their bank.