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I mean, they can, but they literally have a .gov webpage dedicated to Bill Clinton's 2000 act of ending SA and how it will not be changed. So they could, but they'd be renaging on a promise made to the civilian and commercial public. Which in the current climate is entirely possible.
So yes, a receiver could be altered based on a predetermined hard coding, but I think something like that - Geo based hard coding - would be available knowledge, maybe not widely or publicly, but available knowledge.
As of 2007, SA has been disabled in GPS when they updated their transmitters