If you decide to upgrade to a higher level plan during the month, we prorate the amount of credit you've used for the current month and apply the remaining balance of your original plan to the upgrade payment.
Let's say you upgrade from the Developer Plan to the monthly Business Plan ($150), at any time on March 20th UTC. You will only be charged for the 12 out of 31 remaining days in that month for a total of $58.06.
Let's say you purchase the monthly Business Plan on the first of the month for $150. You decide to upgrade to a different plan that costs $3000 any time on March 20th UTC. Your initial $150 payment is prorated proportionally for the first 19 days you've already used for your first plan. $58.06 of your initial $150 payment is credited towards the new plan.