As soon as you've reached 85% ad request usage, you'll see an "Upgrade Plan" button in your portal.
After clicking the confirmation button:
If you wish to upgrade from Essential to Professional, please contact your AdGlare account manager.
Let's say your on a 10M Professional plan at $349/month, and you upgrade to a 25M Enterprise plan at $599/month. The difference between the two plans is $250, which will be charged in a one-time invoice. You don't have to pay this amount manually. Your card on file will be automatically charged 48 hours after upgrading.
Since lower plans have a limited number of features, a downgrade requires an account manager to take a look manually. This way ad serving can be guaranteed, as you may be using ad formats that are not supported in the Essential or Professional plan.