I think it is easiest to do it in the spring/late winter. When you reverse brood boxes, the bottom box will usually be practically empty of bees, brood, eggs etc. That isn't always the case (like this spring) but it typically is. The colony has worked their way to the top leaving the bottom brood box empty. The queen is ramping up, but the population is pretty much contained in the top where the remainder of the food is. If there is brood in the top and bottom, I leave it as is. You don't want to split up the brood pattern.