When the credit hits the closed account, the CC issuer will either transfer the credit to a new account with the same issuer (quite typical when there has been a data breach and a new card with a new # is issued) or issue a check to the CC holder when his statement hits and there is a credit balance (the CC balance having been presumably at $0).

This happens all the time and it's not a big deal. Absolute worst is that your CC issuing bank is not customer-friendly and you actually have to call for the refund.

If you've actually moved since you closed the account, you will want to call with the new address before the credit hits.