When you upgrade your account, new credits will be added immediately. Any unused credits will still be available. If you upgrade from another paid subscription, your billing cycle is reset to the day you upgraded.
It's January 1st and you're currently on the Starter Plan. Your next refill date is on January 10th. You choose to upgrade to the Growth Plan. You immediately receive the new credits and your next refill and billing date will be a month later, on February 1st. All remaining credits and all collected data will of course still be available after your upgrade.
When you downgrade your account, the downgrade is pending until the beginning of the next billing cycle. You'll be able to see your pending downgrade in the Account section here. Unused recording credits will remain on the account for normal plans (Starter, Growth & Pro). Any remaining days of your last subscription cycle will not be refunded.