FMS is quite specific in prohibiting changes. They are specifically prohibited in the LOA and that is standard in the T&Cs.
Like mentioned before, aircraft are not cars—they can’t just swap programming, change configuration, plug in their own kit WITHOUT USG approval.
Not only prohibited by law, such changes require extensive flight testing and verification — finally, there is a whole supply train and support package that can be suspended (and jets need that), and imperiles further deliveries and place at risk FMF funding- and that can really hurt.
Please do not think it is a simple process, legally and technologically. Jets need spare parts and that is another issue that complicates things even further-especially jets make with composites.
I mean you no offense and I am not trying to be argumentative. The fact of the matter is that Israel has done these things in the past and I have every expectation that they will do them again and with the tacit understanding of the US government.