I wish to hang a clg fan where there's an existing plastic box.

I believe the plastic box is supported by a metal between-stud bracket. I
plan to reinforce the box by putting a 2x4 above the box and using lag bolts
w/washers thru the box to support it.

My question would be the THREADS of the box. I'm sure the box itself, as it
will be supported, would well stand the force of the fan; however there's a
big question if the THREADS would have any problem.

One thing about the plastic threads is that you have a large number of them
(sticking the screw up to 1" into threads) as opposed to the steel, which
has only a small number of threads. Anyone have experience with this?

If I need to get the box off, how best to do that? Would I *HAVE* to do it
in the attic? It's VERY hard to get in there and I want to use the existing
hanger, only REINFORCING the box with the new wood. I don't see any
internal screws which hang the fixture