The Department of Justice (DOJ) clearly can reverse its non-enforcement of federal law and begin immediately to arrest sellers and distributors of marijuana irrespective of state laws. Because this would have such dramatic consequences for many who have relied on past practices of non-enforcement by DOJ, the Feds would logically give prior warning even though they are not legally required to do so. And they would presumably- either locally or nationally- establish criteria prioritizing states without medical marijuana laws before launching raids in states where it is a firmly established and widely accepted business.
But the US laws remain on the books for AG Sessions to prosecute aggressively if he so chooses. Many, if not most, federal judges and prosecutors will lack enthusiasm but must still follow the law. Meanwhile, many voters, especially Millennials otherwise bored with current politics, will be energized to oppose these measures.
The marijuana issue was a sleeping time bomb during the 2016 elections getting scant attention as Millennials and others who care about the issue assumed nothing could change so they could ignore it, complain about both candidates and do little about the election. Many may awaken abruptly from this slumber.
Read the Forbes article Coffey is quoted in here.