California Code, Civil Code - CIV § 1698

(a) A contract in writing may be modified by a contract in writing.

(b) A contract in writing may be modified by an oral agreement to the extent that the oral agreement is executed by the parties.

(c) Unless the contract otherwise expressly provides, a contract in writing may be modified by an oral agreement supported by new consideration. The statute of frauds (Section 1624) is required to be satisfied if the contract as modified is within its provisions.

(d) Nothing in this section precludes in an appropriate case the application of rules of law concerning estoppel, oral novation and substitution of a new agreement, rescission of a written contract by an oral agreement, waiver of a provision of a written contract, or oral independent collateral contracts.