Based on my less than perfect back and the experience people have shared
here, it is my intention to convert my hives to medium brood chambers next
year.  I will have both my three existing hives (if the gods are favouring
me and they all make it through the winter) and a few new nucs (which of
course always come on deep frames) (don't they?) to convert.

I've been pondering the best way to accomplish this with minimum impact on
the bees.

Clearly part of the answer will be to install nucs and put overwintered
colonies into single deep boxes, and add mediums full of foundation (above
or below?)  I've also considered making some plywood "Plugs" shaped like
deep frames to put into the deep boxes in place of any empty deep frames,
just to take up space so the bees expand into the mediums rather than
building in the deep.

At some point I will have to remove the last of the deep frames and I'm not
sure how best to accomplish that.  They won't be empty.  For sure I don't
want them to be containing brood.  Perhaps I should at some point move the
deep box with (probably) brood and "Plugs" above a queen excluder.  I
imagine that as the brood emerges the bees will fill their cells with honey.
I could perhaps extract that and feed it back to them.

This doesn't seem terribly straight forward.  Can anybody suggest a simpler
method?

Frank.
-----
The very act of seeking sets something in motion to meet us;
something in the universe, or in the unconscious responds as if
to an invitation.  - Jean Shinoda Bolen