To create an Upsell or a Change Order, follow along with this video to create a new Sale. If new contractual documents are needed, create a Quote first, then import the Approved/Signed Quote to a new Sale.
Note: Some companies use the Sale Type field to track and report on different types of revenue. For example: you may have a Sale Type for "Service" and another Sale Type of "Change Order."
See also: How To - Edit Picklist Values