*

То, о чем Вы спрашиваете, по-английски называется
no oral modification clause.
Формулируют по другому, примерно таким образом
The terms of this Agreement may not be modified other than in a writing signed by the parties.
или посложнее
This Agreement may not be modified other than in a writing signed by both Parties and stating its intent to modify or supersede this Agreement.

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Note added at 11 hrs (2014-05-23 19:40:54 GMT)
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Название такой статьи договора обычно Entire Agreement.
Например
1) Entire Agreement; No Oral Modification.
This is the entire agreement between the parties with respect to the subject matter hereof. Employee represents and warrants that no promise or inducement has been offered or made except as set forth herein and that the consideration stated herein is the sole consideration for this Agreement. Any other promises, written or oral, are replaced by the provisions of this document, and are no longer effective unless they are contained in this document. This Agreement may not be modified other than in a writing executed by both parties and stating its intent to modify or supersede this Agreement.
2) Entire Agreement; Amendment. This Agreement, including the Exhibits, contains the entire Agreement between the parties relating to the subject matter hereof. All prior agreements and all prior negotiations, representations and communications relating to the same subject are superseded by this Agreement. This Agreement may not be modified other than by a written document signed by an authorized representative of each party.
3) Whole Agreement: This Agreement constitutes the entire agreement of the University and the Client and
there are no other promises, obligations or understandings between the parties that are not contained within this
Agreement, unless otherwise indicated herein. The terms of this Agreement may not be modified other than in writing signed by both the University and Client.
4) Entire Agreement. This Agreement constitutes the entire agreement between the Parties and supersedes any prior representations and agreements regarding the subject matter of this Agreement. This Agreement may not be modified, other than in writing signed by both Parties. This Agreement shall not in any manner derogate from the Disclosing Party's rights with respect to any Confidential Information pursuant to applicable law.
Ср
1)Все устные оговорки по настоящему Договору не имеют силы. Договор может быть видоизменен или дополнен только в письменном виде, подписанном обеими сторонами.
2)Любые изменения и дополнения к настоящему договору, кроме изменений и дополнений, предусмотренных настоящим договором, действительны только в том случае, если они оформлены в письменном виде за надлежащими подписями обеих сторон и нотариально удостоверены. Все устные оговорки по настоящему договору юридической силы не имеют.