If you upgrade your subscription in the middle of an ongoing subscription cycle, you will pay the fee for your new subscription plan, and begin a new billing cycle. Any unused credits from your previous plan will be added to your new credit quota.
If you downgrade or cancel your plan, this will not take effect until the end of your current subscription cycle, and you will lose any unused credits on your account when this happens.
Note: Pay As You Go credits are not affected by cancelling or downgrading your subscription. PAYG credits will expire 12 months after purchase.