“Once made, both oral and written contracts may be modified by the mutual assent of the contracting parties, unless third party rights have vested in the interim. As one court explained: ‘A contract can, of course, be subsequently modified with the assent of the parties thereto [citations] provided the same elements essential to the validity of the original contract are present.’ If modified, the original contract ceases to be effective to the extent it is modified, and the rights and obligations of the parties are governed by the modified contract. Mutual assent to a modification requires all the elements of freedom of choice that are required for valid consent to the contract in the first instance.”
[California Affirmative Defenses [certain citations omitted]]
