I disagree. If I get a warning out of the blue, yeah, I take it seriously: something unexpected is going on, and I'm not going to trust the server until I know

I disagree. If I get a warning out of the blue, yeah, I take it seriously: something unexpected is going on, and I'm not going to trust the server until I know what it is. But if I know there's going to be a server upgrade overnight and I get an SSH warning in the morning, I figure odds are good that it's because of the new server rather than a coincidentally timed MITM attack. That's not perfect security, certainly, but as long as MITM attacks are rare it doesn't cost you that much of SSH's value. (All bets are off if someone is targeting your organization specifically, of course: they'd presumably have heard about the server changes in advance and take that opportunity to attack. If I considered that a serious concern in my circumstances then I'd ramp up my security level across the board.)