problemchild wrote:

I'm trying to replace a ceiling fixture. The old box is black metal,
shallow, appears to be intended for bx cable, and I can't figure out
how the heck to get the darn thing out in order to put an old work box
in there. I can't just reuse this box because the center pole juts out
too far below the ceiling level. The box itself is plastered in, and
seems to be held on by the metal brace that runs through it, not above
it. Likewise, the center pole seems to be held on via this thin metal
brace, which extends from either side of the box into the plaster and
woodwork. Can anyone offer suggestions on how to get this thing out
without having to completely rip/cut it out and replaster the entire
thing? Thanks for any suggestions.

Take a screwdriver and carefully remove the plaster where the brace
meets the edges of the box. You should find a 8-32 machine screw on
each side of the box that holds the brace to the box. Once the screws
are removed, carefully chip away enough plaster to remove the brace.