At AdStage, we bill a month in advance. When an auto-upgrade occurs, our payment system processes a refund for the amount that you had left on the current plan. Then it charges the new plan price for the time that you have remaining in the billing cycle.
For instance: If you're on our Standard plan ($499), and your ad spend exceeds $25k, 10 days prior to your bill date, our system will refund you 10 days of the Standard plan back and charge 10 days on the new Professional plan.
This new amount will be taken out at the end of the billing cycle and you will be charged the new plan price from there on out (as long as you stay within your current subscription requirements).