Answer
No — the Contract field on the CustomerOrder logical unit isn't flagged as updatable, so it can't be changed once the record exists, whether through the client UI or by calling the underlying Modify__() API directly via PL/SQL (which will explicitly reject the attempt, citing that the field can't be modified). Any field lacking that update flag is immutable after creation by design, not just in the standard client — re-creating the order on the new site remains the only path for this specific field.