Hey Julius, thanks for getting in touch. You may have seen, Clay doesn't have proration today, in part because all features and credits for the new plan are supplied immediately on payment.
All that said, the previous Starter payment has been canceled, credits removed, and refund submitted. The refund should reflect on your end within 10 days. If you don't see it after 10 days, let us know, and we'll check with our providers.