If your ceiling is drywall and then wood lath & plaster, do you need to change the install? Can the bracket just sit on top of the lath&plaster, or is that going to make it too high?

The dimension from the flange of the can to the duct adaptor is only about 1 1/8” thick. This space must be able to accept the depth of the drywall + lath & plaster + flex-z-fast bracket. With the bracket being ¼” deep the really only leaves 5/8” of thickness left which is usually the thickness of ceiling drywall. The additional layer of lath & plaster is probably going to make the ceiling thickness more than the space allowed. You could avoid this by notching the lath & plaster where it would come into contact with the duct adaptor but you are going to have to be creative.