Booleans tend to produce lots of long, thin facets which Poser won't render nicely - they're best avoided. I'd probably try to do that sort of thing in a modeller that has a knife tool, or something similar - I think most do. Even Anim8or has one. :) However you would then have to fill in the cut edges and UV map them so they would take a texture. It's all a bit time consuming - I know, I've done it myself.

Swapping objects in a scene can be done by rapidly translating them into / out of sight - but why not just split your animation at this point, and splice the two together?