Hi, Stephen.
Swap when you recognise three factors: a colony expanding rapidly, weather that is consistently in double figures, and regular forage.
Expansion will provide nurse & wax bees to fill the void created by the swap; good weather will enable flight, and fresh pollen & nectar will feed the engine of expansion.
Two of these three factors are enough: for example, if expansion produced plenty of young bees and recent flight brought in a lot of fresh pollen and nectar, but now the weather has changed for the worse, the boxes are swapped, because further delay will invite congestion.
I would expect the colony to work on the unused combs - they may be empty, contain old pollen, or a honey arch - and fill the space with brood. If a honey arch is seen in the top box combs before swapping, I would scratch or bruise the cappings to get them to move up the honey.
If the flow is strong or consistent I usually add a super of drawn comb, to deflect nectar up and away from the BBs.
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