When you upgrade your plan, the credits from your current plan will be replaced with the full credit amount included in your new plan, regardless of how much time is left in your current billing cycle.
You’ll be charged for the new plan, but you’ll also receive a proportional refund for the unused portion of your previous plan (based on how much time has passed since your last billing date). In addition, your billing cycle will reset and start from the day you upgrade.
Example:
If you’re on the Starter plan and halfway through your billing cycle, and you upgrade to a Pro plan, your remaining Starter credits will be removed and replaced with the full 4,000 credits from the Pro plan. You’ll be refunded for half of the Starter plan’s cost, and your new billing cycle will begin on the day of the upgrade.
For more information on plan upgrades, visit this article.