To legally change any portion of any type of written agreement, one must look to the original agreement to see if there are terms governing changes. In most written agreements you will find a section that states that changes to the agreement must be in writing (and signed by both parties). This prevents one side from changing the terms and/or conditions of the contract without notification to and agreement from the other side.
The formal term for this is "executing an amendment." A proper amendment will state with specificity the agreement it's modifying, the parties who are doing the modification (which should be the same parties who signed the original agreement), the date of the modification, the specifics of the changes to be made, the number of the amendment, (is it the first? Tenth? Fiftieth?) and a final statement that anything NOT modified in the amendment stays the way it is in the original agreement.
Amendments can also take a few different forms. They can be single or multiple pages (depending on the scope of the change(s)). They can be "form" documents created for regularly-changed documents (like a Change Order to a Statement of Work). And they can also be in the form of a letter, signed and sent by one party but then countersigned by the recipient.
It is lastly important to remember, amendments must be treated as part of the original contract - for once they are executed, it is as if the original contract contains all of the amendments made subsequent (and amendments are applied in the order that they are executed). Forgetting to consider amendments when reviewing contracts is a frequent source of mistakes when interpreting agreements.
This was first published in July 2005