Overage is the cost which you will be charged per-email for each email that you send beyond the credit limit of your current package. Depending on your subscription plan the overage cost per email varies.
Overage fees, if any, will be charged whenever such fees reach $100 or more, or on the monthly billing date, whichever comes first.
Upgrading your account does NOT absorb overage charges already incurred. In case you place an upgrade request when there is already some overage in your account, then those will be first calculated based on your current plan and post that the upgrade request will be accepted.
On upgrade, you will be immediately charged the accumulated overage cost and new subscription cost.