Some contracts are legally required to be in writing. A contract might also contain a clause that explicitly prohibits oral modifications.
But, even in situations where it is possible, it would likely be difficult to enforce.
Any contract—written or verbal—must contain certain elements to be legally binding. To enforce a term that the parties discussed but failed to document, you might struggle to prove the following legal requirements were met:
Offer and acceptance: One party proposed a change and the other parties agreed.
Consideration: All the parties gained something of value from the agreement.
Intention: There was mutual intent to enter into a binding contract.
Legality: None of the contract terms break the law.
Capacity: The parties were mentally and legally capable of entering the agreement.
Without these elements, a court would likely void the amendment.