Convert a Full Depth to a Medium (3/4) hive is easy.
START HERE: The queen is in the FD brood box under the excluder, lots of brood both closed and open.
Place a drawn medium (3/4) box with some honey stores (it may well be the honey super that has come through over winter) on a hive mat on the ground and find the queen in the FD box. Get her to go into the medium box by picking her up (gently) or gently laying the frame she is on flat on top of the medium box with her on the lower surface – she will go to the darkness as fast as anything.
Put the excluder on the FD box, and the new medium box on top of that, and maybe a honey super above that. Probably add the other ¾ box above the one with the queen if she is laying a lot or lots of honey is coming in – do the bees need the space ?
The brood will emerge from the cells in the FD box after a maximum of 24 days (check arithmetic). Then unpack the hive with the medium boxes in the lid, and place the FD box to one side. By now all the brood will have emerged and most bees move up to where the queen is, who is laying. Check some of the FD frames and see if they contain honey (don't worry about pollen). If there is lots of honey, we will arrange “internal robbing” as follows. If not, retire the box.
Place the 2 medium boxes on the base board, put on the queen excluder, and an empty medium box with no frames in it. Then place the FD box on that, and work through it frame by frame, scratching any capped honey with a scratcher – the bees then see this as “free honey” and should move it down to the hive below the excluder. Give them a couple of days, then remove the FD box after shaking off any bees, and retire it, and remove the excluder.
The extra vacant space gives the bees the impression that the wets are not part of their own hive and rob it out and take the honey back down to their frames. This generally leaves the frames pretty dry.
By this stage the new 2 box OSB hive may need another box depending on how the queen is laying.
If you don't have drawn ¾ frames for START HERE above:
A box of foundation may not be used by the bees, but if the queen is there I would be confident that the bees start drawing immediately as long as there is some flow, or you feed them.
However, safer would be to remove from the FD box a couple of frames without brood, and put 2 3/4 frames with foundation just next to the brood frames, on each side. The bees will probably draw those frames quickly, and you can use them in the 3/4 box with foundation frames to draw the bees up. And there will be cells for the queen to lay in straight away. And put back the FD frames you took out.
So it puts you back a week or so, no long time.
And I would do this alteration in spring when the hive is active, not in autumn when it is closing now (like now).
good luck, Tudor - and keep asking if you need more help.