I think it would be best to do that sort of thing when you can prepare a good chunk of your populace to really try it out. "Hey everyone, start driving on the left on your morning commute tomorrow!" That way, you have a concerted effort to do it right, and lots of people driving on one side or the other would reinforce the behavior. That's all as opposed to having a few bleary-eyed drivers in the wee hours potentially get a surprise.