How should the United States undergo metrification?

Technically, that's a loaded question. A better one would be "Should the US undergo metrification?" to which I would answer with a resounding yes. As to how to go about it..... Start with the highway system; that's an easy first step. From there, just progress into other industries.

Jam it back in, in the dark.

What car doesn't have KPH on the scale as well? Every one I've ever driven has - Chevys, Toyotas, Oldsmobiles, Saturns - everything has had KPH right there on the dashboard.

Also, saying "it's always been done this way and if we change business x will go out of business" is never a good reason to do something. The world changes. Businesses that can't adapt fail. It's that simple.

Also, replacing all the signs on the highway may be a reasonable cost (a billion max - pennies for the feds) but it's a one-time cost.

There's nowhere I can't reach.