Answer:
Midstream changes (are unenforceable).
Step-by-step explanation:
In the private sector, the preexisting duty rule often arises when one of the Parties to an existing contract seeks to change the terms of the contract during the course of its performance.
Such midstream changes are unenforceable: The parties have a preexisting duty to perform according to the original terms of the contract.
What could happen is If a party of a contract runs into unforeseen difficulties while performing it's contractual duties, and have to modify their contract to accommodate this difficulties, the modification will be enforced even though it's not supported by new consideration.