You are already enrolled in usage-based billing if you created a trial of {% data variables.product.prodname_ghe_cloud %} on or after August 1, 2024.
If you currently pay for your {% data variables.product.prodname_enterprise %} licenses by invoice with a volume, subscription, or prepaid agreement, you will continue to be billed in this way until your agreement expires. At renewal, you have the option to switch to the metered billing model.