Though the payment term of CSP is monthly, when customer purchase licenses through CSP the subscription term is actually 1 year. Within 1 year the price and SKU customer ordered will not change, so even if a license is retired the customer can still continue to add/remove licenses until the end of the 1-year subscription term. At the moment there is no penalty if a customer cancels the subscription before the 1-year subscription expiry date. The renewal anniversary is 1 year from the subscription start date, which partner can determine through customer’s O365 admin portal.