When I bought the Yorktown pediments to spruce up the windows, I also bought the Yorktown Front Door by Houseworks. It is the same height as the door that came with the kit, but the opening wasn't wide enough to accommodate the new door. Before making any cuts on the prefinished brick piece, I cut a mockup out of foam core board to try out the positioning. I decided to widen the opening toward the outer edge since I didn't want the door any closer to the window.
I used the utility knife technique shown on dhbuilder.com. First, I scored the line on the brick side using a straight edge, cutting toward the opening to avoid accidentally scratching the remaining brick surface. After reaching some depth in the brick surface layer, I flipped the board over and started cutting the back side, chipping out the mdf along the way. Then, I continued working on the front chipping out the brick layer and mdf underneath until the section began to separate. The small portions of brick that came off on the remaining wall will be covered by the door trim.
I kept the brick scraps in case I need to repair any damage done during construction or in the future.
The new front door fits into the enlarged space nicely. I will most likely add pieces under the bottom trim to prop the door up in the space when it is installed; this will make sure the top of the door and the top of the adjacent window are level. It might not be like that in real life, but in this small of a space the horizontal lines of the brick make any difference in height obvious.