If the replaced area of composite is going to be relatively large (more than a couple of square meters or so) I will be quite amazed if they can just brush a bit of resin on a scarfed joint and call it good to go.
Even if it is scarfed, since there will be no carbon fibers running through the glued joint, it seems difficult to believe that just a resin bond will stand up to 20 or more years of flexing and pressure cycles. I would think they would need the composite equivalent of a doubler plate across the joint to be sure.
I don't remember exactly why, but I seem to remember that for some of the very early build 787's weren't some of the fuselages disassembled and reassembled (maybe even more than once)? In that case, it seems clear that the fuselage barrels can be replaced if necessary. It may mean some new connectors in the electrical cables and hydraulic lines, but there are already plenty of those on the airplane.
Of course Boeing is very keen to show the world that it is not too difficult to repair even a large area of composite. But I really hope that they are not going to regret their decision to patch the airplane instead of replace the whole barrel.