The comment you replied to never claimed that the U.S. uses metric units, only that it has been legal and even encouraged since 1866. So to re-iterate, there no

The comment you replied to never claimed that the U.S. uses metric units, only that it has been legal and even encouraged since 1866. So to re-iterate, there no change needed to the law, it already allows metric just fine (as you yourself note when you mention that the government-run military understands metric). Simply deprecating customary units won't be enough to make the switch, you would actually have to make it illegal, and Hell will freeze over before you'll see that pass in the U.S. ("Doesn't Congress have actual problems to fix???")