[digitalPhonix]

A lot of the commonwealth countries switch ~1970 (UK, Australia, Canada at least).

Why does the absolute number matter? Every trades truck/stockpile will need to stock double for some time, not some absolute value increase.

[xp84]

To some extent yes, but I just think it would be a greater hit to the overall GDP with more mechanised things in existence. Maybe I'm wrong, I just think that in the farther back you go, the greater the share of the economy that wouldn't be dramatically impacted by a metric changeover. Like, farming was somewhat a greater share of the GDP then than now, and a lot of goods are weighed and sold in bulk, not really all that hard. A farm would need to replace irrigation pipe and retrofit their scales, but not demolish and build new farms. A car factory, on the other hand... it would take a while for all those machines to naturally be replaced.