It _can_ be done piecemeal. Weights and measures (except for pints of beer) are now supposed to be metric in the UK.

Metric's better -- a single measure got bigger .

Distances and speeds are still in miles and mph. Investing in changing road signs will be a major issue. One which will actually cost the government real money. With the weights and measures, the supermarkets bore the brunt.

Of course, persuading people to drive on the wrong side of the road will take a lot more effort.