When you choose to apply new siding over existing siding, you save money early on but are in for a real treat in the future. The Picture shows rotten wood in the corners because of homemade wide j-channel bridging old and new siding. Wider flashings like this can tip back into the window creating a water path. Installing future windows in a situation like this will not allow them to be flashed per code and ultimately lead to a recurring situation. Cost is more for complete tear off. Piece of mind knowing you have new house wrap, standard flashing tape, and the possibility for future openings to be done correctly is far better than the alternative of mold, mildew, and rot.