In the interests of learning how stuff works, you might as well turn it inside out and have a look. It is possible to figure out how all the seams add up, and how it's been made.
When I've done this, my conclusions is usually that it's very difficult to retrofit pieces, because you would, in most cases, have to unpick the seams and sew them together again, to take the strain off the old fabric and transfer it to the new fabric. Internal seams are usually trimmed quite thin, so there may not be much leftover material to do it any other way.
Anyway - to save further waffle, just get yourpack and turn it inside out, and have a look.