Going from double brood to a single box for winter in Drenthe — which box do I keep and where does the surplus honey go?

Nine years, twelve colonies, all on Simplex double brood through the summer. Most of my neighbours here winter on a single box plus a super of stores, and after two damp winters where the bottom box came out mouldy and empty I'm inclined to try it. The practical questions I keep going round in circles on. First, which box: usually the cluster and the remaining brood are in the top box by now, and the bottom is dark empty comb with a bit of pollen. Keeping the top box means moving it down onto the floor, which is easy enough on a cold day. Second, what do I do with the frames I take out — some have a couple of kilos of sealed stores in them and it seems mad to store them when the colony might need them. Third, and this is the one I really want opinions on: does a colony on a single box actually winter better here, or does it just mean less spare food and a tighter margin if February drags on? Our winters are mild and wet rather than cold, so I suspect condensation matters more than volume.