I was forced to change to the metric system in
the early 70's(that's when New Zealand converted).This is basically what happened.
A pint of milk became 600ml.(the american Gallon was always
devised by the rich to steal from the poor.)
The currency had changed from Pounds to Dollars in 1967
and 1 pound became $2.
Tradesman bought themselves a new Measuring tape.The smart ones with Metric only measurements.
The schoolteachers were already trained in the Metric system.
As a motor mechanic the toolbox had to be topped up but it was coming anyway because the Japanese had just started making the best cars around.
The biggest costs were in signage and tooling for industry.
The academics are probably still standing around scratching their Balls but it was a very simple transistion
for most.