Hi everybody, this is my first post.

I'd like to replace our front door with a pre-hung model with a full-size light. The hitch is that the slab door in question seems to occupy a pretty tight opening: Not only is the trim covering its rough opening only about an inch and a half wide, but there is a window right above the door -- there's only a 1-inch board separating the top of the door and the bottom of the window. (The door is set back pretty far from the window, though. The one-inch plank between the top of the door and the transom's window glass forms a shelf about 10-inches deep.)

I've uploaded pictures of the door/transom and the jamb here: Renovations - a set on Flickr

Can anyone give me advice on how to replace a door in a tight spot like this? Can I just pry off the existing casing, measure the rough opening, and order up a pre-hung door fitting those dimensions? Is there anything special I need to do to attach the top of the frame to that funny "shelf" below the transom?

thanks in advance for your help,