2005 California Civil Code Sections 1697-1701 CHAPTER 3. MODIFICATION AND CANCELLATION

1697.  A contract not in writing may be modified in any respect by
consent of the parties, in writing, without a new consideration, and
is extinguished thereby to the extent of the modification.
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.
1699.  The destruction or cancellation of a written contract, or of
the signature of the parties liable thereon, with intent to
extinguish the obligation thereof, extinguishes it as to all the
parties consenting to the act.
1700.  The intentional destruction, cancellation, or material
alteration of a written contract, by a party entitled to any benefit
under it, or with his consent, extinguishes all the executory
obligations of the contract in his favor, against parties who do not
consent to the act.
1701.  Where a contract is executed in duplicate, an alteration or
destruction of one copy, while the other exists, is not within the
provisions of the last section.