Each service plan has the list of allowed upgrades and downgrades. The list defines service plans that customers (resellers) are allowed to change their subscription service plan to. 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, they are billed for services that were added to their subscription, if they switch to a cheaper plan, they may be refunded with the appropriate sum.
In case resource upgrades are configured for a service plan a subscription is based on, then resources can be upgraded for the existing subscription.
In case resource rates are configured for a service plan a subscription is based on, then resource limits can be upgraded or downgraded for the subscription.
If you configure OA Billing to delay the processing of Change Orders which 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.