Each service plan has a list of allowed upgrades and downgrades. The list defines service plans to which customers (or resellers) are allowed to change their subscription to the service plan. Generally, both upgrade and downgrade procedures are switching subscription service plan to another. The difference is that when a customer switches to a more expensive service plan, the customer is billed for additional services; when a customer switches to a cheaper service plan, the customer may receive a refund (depending on the service plan settings).
If for a service plan resource upgrades are configured, then for a subscription to this service plan resources can be upgraded.
If for a service plan resource rates are configured, then for a subscription to this service plan resource limits can be upgraded or downgraded.
If you configure Billing to delay the processing of Change Orders that contain downgrade items, such orders will be processed on the next billing date. For more information, see Configuring Delayed Processing of Orders Containing Downgrade Items and Delayed Downgrade Scenarios.