When you apply a change request to a contract, the changes on that change request are applied to the active contract. The change request then becomes a superseded contract and contains the contract as it was before the change request was applied. In addition:
Billing schedules that have not been billed are deleted.
Draft billing documents for the contract are deleted or discarded depending on the Allow Deletion of Draft Billing Document setting in the Billing Central Settings custom setting. See Allow Deletion of Draft Billing Document.