While pre-hung is the industry standard for all doors, both interior & exterior. The trim is installed after the paint unless it's paint grade and painted the same color as the walls. Trim is easy to replace, just pop it off and put fresh up. if it's the door jamb itself it gets a bit more anoying. then you'll have to remove the door from the jamb, pop the trim off and tear the jamb out. You may be able to purchase a door jamb from a local suplier, but it's going to have to be one that carries only doors and windows. The ones that carry everything generaly won't have just the jamb. The suplier should have a sales rep come out & look at the door to make sure that he orders the right jamb for your application. It will arive pre drilled & routed for the hinges as most carpenters don't carry a door jig anymore. now you'll need to install the hinges & door in the jamb & hang the door. Reattach the trim and your done. best of luck to you :)