It's really bad for your car to have braided on one part and rubber on the other, puts too much stress on the rubber and they could burst, I would get the rear ones ASAP!
As for changing lines it's ridiculously simple. Each line is connected with one bolt on the caliper and just a screw on fitting to the body. You need to undo that then install the new ones, then bleed your brakes to get all the air out of the lines.