There are three ways an existing treaty can be amended. First, formal amendment requires State parties to the treaty to go through the ratification process all over again. The re-negotiation of treaty provisions can be long and protracted, and often some parties to the original treaty will not become parties to the amended treaty. When determining the legal obligations of states, one party to the original treaty and one a party to the amended treaty, the states will only be bound by the terms they both agreed upon. Treaties can also be amended informally by the treaty executive council when the changes are only procedural, technical change in customary international law can also amend a treaty, where state behavior evinces a new interpretation of the legal obligations under the treaty. Minor corrections to a treaty may be adopted by a procès-verbal; but a procès-verbal is generally reserved for changes to rectify obvious errors in the text adopted, i.e. where the text adopted does not correctly reflect the intention of the parties adopting it.
Parties to an original treaty and an amended treaty are bound to what terms?
the terms they both agreed upon