I honestly do not think this will ever happen to be honest because its just simply not possible in my eyes.
The dollar is way to strong to take over..its globally so strong the bitcoin is nothing compared with it.
One day it may occur that the true money tries to compromise with the fiat money. If this happens the dollar might be replaced by bitcoin. It takes centuries for that particular day to come. When people start to use bitcoin in all places that might be the start of such a scenario to take place in the future...