Sounds like a split-jamb prehung door. I cannot think of an instance in which you would not be able to take the casing off, based on what you've explained. My only guess is that your builder is assuming the prehung is the kind where the casings are stapled at the factory and not nailed - I've seen those, but still, if you are careful, it can be done, especially if the jambs are wood, a little repair may be in order after the fact. Another consideration to make is your baseboard molding. How do the baseboards mate up to this door? It would be at least worth noting that replacing the existing trim with something that is going to match up exactly where the old baseboards did might not be the easiest thing in the world. As stated already, a picture would be a big help in helping you.