A pistol can change to a rifle, but not back, a rifle can never change to a pistol.
Why can't you change it back? Where did you get this info from? I've never seen anything to indicate a pistol can't be converted to a rifle and then back to a pistol. I agree on the rifle first can't go to a pistol.
edit to add: When somebody uses a Mectech (sp?) conversion for their Glock, do you think they are stuck that way?