When a monster decides whether you are peaceful and makes race or alignment checks to do so, they do it based on polyform, current alignment, and when they first spot you as opposed to when they are first created. If the player polymorphs into an orc and some other orcs are hanging out in an unexplored room across the level, they should be peaceful to the character as long as the character is polymorphed into an orc (but will turn hostile when she changes back).