I'm looking to do the same thing. I will be splitting my two double deep hives, ending up with four single deeps. I'll put mediums on top as they need them, and once the queen is laying in two mediums above the deep, I will pull the deep out, shake/brush _ALL_ the bees from the deep into the mediums (now on the bottom board), put a queen excluder on and then put the deep on top. The queen should now be down below the excluder, and nurse bees will come up to tend the brood in the deep. Once the deep brood all emerge, I will add a medium under it, remove the excluder, and use the deep for honey, and remove it with a harvest.
I should then end up with 3-medium brood chambers, about the same height as two deeps. My bee-cozy's will fit nicely.
At least that is my plan. I am a second year beekeeper, so this may not be the right/best way to do it.
Cheers,
Grid.