A Bailey Comb exchange is an excellent technique that can be used for a couple of reasons:-
1) Replacement of old brood frames with new. If you haven't gotten around to replacing your comb for some time and the comb in the frames are black and/or have excess drone brood or holes, a one-time change may be the answer instead of wasting brood with a shook swarm or changing just a few frames at a time.
2) Moving bees onto different sized frames. You might have purchased a colony on standard sized frames Deep Nationals (DN frames) and want to move to say, 14 x 12's or Commercials.
A good time to do this is in spring before you need to put a super on the hive, or in late summer after the supers have come off whilst the weather is still warm.
The procedure is quite simple. You'll need a brood box with the new frames in, as well as a queen excluder. The frames can be of foundation or clean drawn comb. An eke is also beneficial to make a top entrance. And if you use an eke, you'll need an entrance block or some means of closing the bottom entrance when the top one is being used. An eke is a rectangular frame of wood - say 25 or 50 mm high - that can form a spacer between the two brood boxes. In it a hole or slot is made in one side to allow the bees to fly. Ekes can be made easily enough if you are OK at DIY or they can be purchased from beekeeping suppliers. If you don't want to use an eke, you can omit it from these instructions.
This is what you do. (Note there are shorter methods than this but there's no rush, - let's work with the bees!). Note that a weak colony will struggle, so you might consider allowing it to strengthen first and certainly don't try this with a weak colony in cold weather.
1) Locate the queen on a frame of brood and put it in a new brood box which is resting on an upturned hive roof.
2) Close up the gap in the old brood box and place a queen excluder on it. (The brood box remains on the hive floor).
3) Place the new brood box with frames on top of the old brood box you want to replace. The frame with the queen needs to be over existing brood below the excluder which will help keep the brood and queen warm. If you are moving to 14x 12 or commercial frames, you can still use this technique as a national deep frame will fit. Place a crown board on top and a feeder on that, surrounded by an empty super. Put some insulation around the feeder to reduce heat-loss. (This can be bubble-wrap, scrunched up newspaper or old towels or whatever you have). Leave them for a week - continue feeding with 1:1 syrup if the feeder becomes empty, as hopefully the bees will realise there's food above and use it and start to draw comb. If they don't like a rapid or Ashforth type feeder, a contact feeder can work better in some cases.
This is the configuration you will start with:-
Floor; Brood box with brood and bees and no queen; Queen excluder; New brood box with queen on a frame of brood surrounded by drawn comb ideally or foundation if none available; Crown Board; Feeder with empty super around; roof.
4) After a week, we would hope that there is drawn comb in the top brood box and the queen is laying in it. Only once there is brood (larvae, not just eggs) should the old brood frame be removed to the lower brood box. The reason for this is that bees will abandon a queen if the weather is cold but they won't abandon brood. Leave the old brood frame above the excluder for another week if you need to.
If there is plenty of nectar coming in, you may not need to feed over the next few weeks however the bees could have a lot of work to do to draw foundation so feeding definitely helps this process ans stops bees from robbing wax from foundation that you have just put in (They do that sometimes). When you inspect shortly after starting the comb exchange, you may be disappointed that there is not wall-to-wall brood in the top box but remember that the bulk of the bees are keeping the brood warm in the lower box so it can take a while for the process to really get going; you need to wait for the brood to emerge before the bees will all move up. You should be able to find the queen fairly easily at this time as there is usually not a huge number of bees in the top box.
5) Once the queen is on new brood in the top box, it's time to add the eke over the queen excluder. The reason for an eke is to get the bees to fly directly into the top (clean) brood box rather than walk over the old and dirty comb. The bees won't abandon the brood below and the queen won't walk out of the newly-formed entrance and unless you do this in a high-wasp environment in autumn, the bees can defend the colony. The flyers are now able to enter by the top entrance in the eke and you now need to close off the lower entrance. It will take a little while for the bees to work out that they need to use the top entrance, but they will soon enough.
So in order, you will now have:-
Closed off floor; Brood box with brood and bees and no queen; Queen excluder; Eke and entrance; New brood box with some drawn comb and queen; crown board; feeder with empty super around; roof.
6) The colony need no more manipulation until all the brood has emerged in the lower box. This will be 3 weeks from when you moved (a frame of) brood down to the lower box. Bees like to have their stores above and behind them so with the top entrance, the bees will bring new food into the clean brood box and they will also tend to remove stores from the lower brood box and carry them up through the excluder. As you do your weekly inspections, you might like to release any drones if they are trapped below the excluder.
7) After the three weeks has passed (add 3 more days if there is drone brood) and there is no brood and probably little food in the lower box, it can be removed along with the eke and queen excluder and the lower entrance restored. If there is more foundation to draw, you may wish to (continue to) feed - and in autumn you will want the hive to be full of stores in any case.
Note 1: If you have stores in a few frames that need clearing out, (say, some set OSR honey in super frames), having the food below the brood results in the bees removing it and placing it where they want it, so you can clear out old supers in this way - under the brood.
Note 2: If you are using polystyrene hives, you won't need insulation around the feeder.
What to do with old comb? It's tempting to keep it 'just in case' but that will usually mean that you use it yet again in an emergency so the best thing is to melt it down or dispose of it as soon as possible! You might want to keep a couple of the better frames for use in a bait hive. Decent old comb can be sterilised with Ethanoic (Acetic) Acid. Old comb can be useful as a firelighter for a fire or chimnea or fire-pit. Very old combs will give you almost no wax and a lot of mess trying to obtain it so it's probably not worth the bother trying to do much with it. However it can be cut out of the frames and the frames cleaned and re-used.