I can't imagine a situation where an actual contract addendum would ever be necessary or even prudent for this type of thing. <SNIP>
A waiver or deviation--however those terms are defined--(a "concession") doesn't require changing the contract because if it's properly documented (almost always using the customer's methods and forms), the specifications are officially changed. A contract typically calls for performance to the specifications, most of which are specified by reference. It makes no sense that there would need to be a contract addendum when the contract terms haven't changed.
A waiver or deviation--however those terms are defined--(a "concession") doesn't require changing the contract because if it's properly documented (almost always using the customer's methods and forms), the specifications are officially changed. A contract typically calls for performance to the specifications, most of which are specified by reference. It makes no sense that there would need to be a contract addendum when the contract terms haven't changed.