Not much happens at all. A small group of very large beeks will show up at your door and just slap you around a little. It's only after the second violation that they drag out the waterboard. The purpose of the guidelines (not rules) is to help you get your hives prepared for the coming winter by allowing the bees to build up drawn comb for honey storage and brood. If you add the next box early the bees are likely to move up and not draw out the remaining frames in the lower box. Plus, they may not be expanded, number-wise, enough to handle extra empty space. It's your call!