Seeing as the material is listed as a critical characteristic, it's going to take a little more than, "Yeah, let's do it!" Mainly, because I would think the part number would roll (you would have to have completely new heat treatment requirements, as 9310 requires different heat treatment).

Changing a non-critical pistol grip panel from Nylon 6/6 to 6/12, or its color, it a lot easier.

There is also the case where the contractor calls you up on the phone and says, "Oh, by the way, we've changed the material of the spring guide from aluminum to plastic. Hope you don't mind." Then the Govt has to show a technically valid reason why that change is unacceptable, or just accept it. One of the downsides to not owning the design.