I was in Germany reading the stories on the Swedish switchover...from memory, everyone was ordered to get off the roads for a period of reflection, then the whole system re-opened...I always wondered what the casualty count was until people got used to the new way!
It's hard to imagine getting it right the next morning before having any coffee.
I never went to Sweden, but in those years I was happily driving British cars in the US, American car in England, while using metric and imperial speedometers in all the wrong places, and anything I could get in Spain, where the preferred lane was the middle, saving lots of needless confusion.