It would never willingly or knowingly change its goals from its original ones ... Goals are static.
AIs of the required caliber do not exist (yet). Therefore we cannot see the territory, all we are doing is using our imagination to draw maps which may or may not resemble the future territory.
These maps (or models) are based on certain assumptions. In this particular case your map assumes that AI goals are immutable. That is an assumption of this particular map/model, it does not derive from any empirical reality.
If you want to argue that in your map/model of an AI the goals are immutable, fine. However they are immutable because you assumed them so and for no other reason.
If you want to argue that in reality the AI's goals are immutable because there is a law of nature or logic or something else that requires it -- show me the law.
A stub on a point that's come up recently.
If I owned a paperclip factory, and casually told my foreman to improve efficiency while I'm away, and he planned a takeover of the country, aiming to devote its entire economy to paperclip manufacturing (apart from the armament factories he needed to invade neighbouring countries and steal their iron mines)... then I'd conclude that my foreman was an idiot (or being wilfully idiotic). He obviously had no idea what I meant. And if he misunderstood me so egregiously, he's certainly not a threat: he's unlikely to reason his way out of a paper bag, let alone to any position of power.
If I owned a paperclip factory, and casually programmed my superintelligent AI to improve efficiency while I'm away, and it planned a takeover of the country... then I can't conclude that the AI is an idiot. It is following its programming. Unlike a human that behaved the same way, it probably knows exactly what I meant to program in. It just doesn't care: it follows its programming, not its knowledge about what its programming is "meant" to be (unless we've successfully programmed in "do what I mean", which is basically the whole of the challenge). We can't therefore conclude that it's incompetent, unable to understand human reasoning, or likely to fail.
We can't reason by analogy with humans. When AIs behave like idiot savants with respect to their motivations, we can't deduce that they're idiots.