I'd be shocked if someone greenlighted an atc upgrade procedure which involves closing all US airspace to do the switchover.

That’s obviously not what’s happening. But if this upgrade is really implicated, the real question would be where the line of accepted residual risk for an outage like this was drawn.

Sorry I was unclear; the guess is that it caused an issue, not that they did this on purpose with no warning.

Well sure, but I wouldn't be shocked at all if someone greenlit a plan that wasn't supposed to involve closing the airspace, but which failed and which had an insufficiently thought out rollback plan.