Högertrafikomläggningen happened on September 3rd 1967. On that day Sweden changed from driving on the left side of the road to the right (normalizing with all of its neighbors).
I would guess that before that day many cars in Sweden had their steering wheels on the right side of the car. So what happened to all those cars after the switch? On one side it would be prohibitively expensive to have everyone's car retrofitted with the new direction, but on the other side its more difficult and dangerous to drive on the right hand side of the road with a right hand side steering wheel.
Bonus points: What about buses? They presumably had exits on one side. How did they continue to operate after the switch?