When you cancel your Clay plan, you keep your credits until the end of your current billing cycle.
After your billing cycle refreshes, your credit balance gets capped at 2x the monthly credit limit of your new plan (which becomes the free plan after cancellation). For example, if you cancel today, your credit balance will be capped at 200 credits on the next billing date, since free plans allow 100 credits monthly.
So canceling doesn't immediately remove your credits - you have one month to use any excess credits before they're reduced to the new plan's rollover limit.