Cost-Based Proration will charge customers the difference between the Download they currently have a subscription for, and the price of the Download they wish to upgrade to.
Example: A customer purchased a Single Site license for Product A for $29 on October 1, 2019. On March 1, 2020, the customer upgrades their license to 10 Sites which is $99. The customer will pay $70 to upgrade. On October 1, 2020, the subscription will renew and the customer will pay $99 for Product A (10 Sites) and have an active license key until October 1, 2021.