not sure how that is a sunk cost fallacy. It sounds like you just described a real cost that would be incurred if switching to Linux.
aversion to real costs is not a fallacy.
aversion to real costs is not a fallacy.
I'm guessing that in this example, it was more expensive to switch to Linux and recertify. I doubt anyone in management was really arguing against a cheaper option because of money spent 10 years ago.