Thus, a simple letter addendum stating
"Contract ABC-123 rev 0.1.45 paragraph 1.2.3.4.5.6.7 dated 1 April 2008 requires the use of ASME Code 2007 Div I. Unless otherwise directed, the following pressure vessels per ABC-123 shall be designed, inspected, constructed and installed IAW the latest approved ASME Code for Pressure Vessels."
Do not require anything more elaborate, do not attract attention to the letter nor the addendum, do NOT ask for a contract change NOR ANY FUNDING INCREASE (and not any decrease either) nor a change in schedule. Just a simple, able-to-be-filed-away paper copy letter that goes in the certification folder.
In other words, let the governemnt "decide" to do what is right and proper and conservative in design and safety for the public by "allowing" them to make a decision by doing - and saying! - nothing.