At AdStage, we bill in advance. When an auto-upgrade occurs, our payment system finds the differences between the amount you've paid for your initial package and the amount of time on your new upgraded package. This amount will be added to your next invoice.
If you choose to not renew, any pending prorations will be collected at the end of the period.
For instance: If you're on our Pro plan ($1,499), and your ad spend exceeds $150k 2 months prior to your bill date, our system will add the difference between 2 months of the Elite plan ($2,499) and 2 months of the Pro Plan to your next invoice, in 2 months.
Moving forward you will be charged the new Elite plan price from there on out (as long as you stay within the subscription requirements).