Art. 29 CISG

(1) A contract may be modified or terminated by the mere agreement of the parties.

(2) A contract in writing which contains a provision requiring any modification or termination by agreement to be in writing may not be otherwise modified or terminated by agreement. However, a party may be precluded by his conduct from asserting such a provision to the extent that the other party has relied on that conduct.

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Secretariat Commentary

The Secretariat Commentary on Art. 27 of the 1978 New York Draft, predecessor to Art. 29 CISG, gives insight into the drafters' unterstanding on the eve of the 1980 Vienna Diplomatic Conference.

Commentary on the Draft Convention on Contracts for the International Sale of Goods, prepared by the Secretariat (UN Doc. A/CONF.97/5), in: United Nations Conference on Contracts for the International Sale of Goods, Vienna, 10 March – 11 April 1980, Official Records: Documents of the Conference and Summary Records of the Plenary Meetings and of the Meetings of the Main Committees, New York: United Nations (1991), pp. 14 et seq., at pp. 27—28

Overview of case law on Art. 29 CISG: UNCITRAL Digest

The UNCITRAL Digest on the CISG can be used as a starting point to discover the relevant case law on this article.

2016 UNCITRAL Digest of case law on Art. 29 CISG

Interpretation of Art. 29 CISG: Selected issues discussed in case law

1. Do contract modifications under Art. 29(1) CISG require consideration?

no:

Shuttle Packaging Systems, L.L.C. v. Jacob Tsonakis, INA S.A. et al.
U.S. District Court for the Western District of Michigan
USA, 17 December 2001 – 1:01-CV-691, CISG-online 773

Under the Convention, a contract for the sale of goods may be modified without consideration for the modification

Norfolk Southern Railway Co. v. Power Source Supply, Inc.
U.S. District Court for the Western District of Pennsylvania
USA, 25 July 2008 – 07-140-JJf, CISG-online 1776

"The Parties' agreement went through numerous iterations prior to delivery of the locomotives and equipment. Although not required by the CISG, see CISG art. 29(1), each of these changes was supported by consideration [...]"

New World Trading Co. v. 2 Feet Productions, Inc.
U.S. District Court for the Southern District of New York
USA, 16 May 2014 – 11 Civ. 6219 (SAS) / 13 Civ. 1251 (SAS), CISG-online 2726

Zodiac Seats US LLC v. Synergy Aerospace Corp.
U.S. District Court for the Eastern District of Texas
USA, 08 April 2020 – 4:17-cv-00410, CISG-online 5172

Held that "contract modifications need not be supported by independent consideration to be valid under the CISG."

yes:

VLM Food Trading Int'l, Inc. v. Illinois Trading Comp. et al.
U.S. Court of Appeals (7th Circuit)
USA, 21 January 2016 – 14-2776, CISG-online 2811

Hellenic Petroleum LLC v. Elbow River Marketing Ltd.
U.S. District Court for the Eastern District of California
USA, 18 November 2019 – 1:19-cv-00483-LJO-SKO, CISG-online 4706

Consideration requirement under the Uniform Commercial Code (UCC) applied to CISG contract

2. Are contract modifications that result in a contracting party being replaced by a new contracting party governed by Art. 29 CISG?

yes:

RTM Group Inc. v. Oferol S.A. et al.
Tribunal de Apelaciones en lo Civil de Cuarto Turno (Court of Appeal for the Fourth Circuit)
Uruguay, 27 November 2013 – DFA-0009-000542/2013 SEF-0009-000217/2013, CISG-online 4807