Australia managed to do a complete† change over starting in the 1970s:

* https://en.wikipedia.org/wiki/Metrication_in_Australia

It's going to take decades, mostly waiting for the non-metric folks to die off probably and the raised-on-metric folks to grow up, but if you don't start you'll never finish.

† Can Aussie HNer confirm?

I'm Australian and the only things measured in imperial units is babies (stones? But I think younger parents now use metric, but my parents only know baby weights in stones), beer (pints, but not the same as overseas I think, also it can vary by state), certain types of land units ("acerage" is a common land plot size) and height (feet and inches, for some reason people don't understand it in metric)

These are all customary usage so I'd say we are completely switched over, we couldn't tell you generally what these units look like outside of "how big is a baby" or "how tall is that person" or "is my beer large or small" really.