Recently, I was speaking with a beekeeper in Missouri about why bees move up into the super during the winter months. It's a great discussion matter that every beekeeper should understand.
The first reason for this migration is simple physics: warmth rises. As the heat the bees create moves to the top of the hive, the cluster follows to conserve energy and maintain efficiency. Secondly, honey acts as an insulation barrier; its density prevents rapid temperature changes, offering better heating efficiency.
Finally, the honey is their winter food source. It is much easier for the bees to feed when they are right next to the stores, rather than moving away from the warmth of the cluster.
January is when your bees will be heavily consuming their stored honey. An easy way to check reserves is to pick up the rear of the hive. If it feels heavy, they have adequate reserves. If it feels light, you must start feeding sugar water or supplements immediately.
A critical warning: Beekeepers should not try to force the cluster back down to the brood box. Doing so forces the bees to work significantly harder to stay warm, and you may lose the entire colony.