Even if the US changed every single speed limit sign tomorrow to from MPH to KPH, how hard is it to match a number on a guage in front of you to the sign posted on the road?
US cars have mph as the main unit and km/h as the secondary. Canadian cars, for example, have km/h as the primary and mph as the secondary.
I would never have thought this was an issue until that stretch of time where I was switching between a Canadian and US vehicle for a period of time... it's not rocket science, but it's definitely an extra cognitive load when you're driving, and if you're not used to a particular vehicle then the difference between 55mph and 55km/h isn't as obvious as you'd expect.
It's a bit like what happens when the instrument cluster is put in the center of the dash (fuck the Nissan X-Trail) or the speedometer range is substantially different from what you're used to (if you're used to 100km/h being right at the top of the dial and you move to a vehicle where 80km/h is at the top, you *do* drive slower until you compensate).
As I said, not rocket science, and individually it's not a big issue, but with the sheer quantity of marginal drivers on the roads... I don't expect the transition would be bloodless.