If you cancel or downgrade, your credit balance will remain on your account until the end of your current billing cycle. Once your billing cycle refreshes, you will get downgraded to the new plan, and your credit balance will be capped at 2x of the monthly credit limit according to your new plan. (For example, if you cancel your plan today, your credit balance on the next billing date will be capped at 200, which is twice the monthly credits for free plans.)
You can read more about rollover here:https://www.clay.com/blog/introducing-clay-pricing-3-0-the-most-flexible-credit-system-on-the-market