You'll find yourself with a lower part of the now cut bag made of two distinct entities : the inside of the bag cut in half, and the lower (front) pocket.



Unstitch the upper part maintaining said pocket (fig. 3), then retrieve the canvas separating the two parts (between the backside of the front pocket and the frontside of the main bag) for further use. Make sure you keep it intact.



The goal is to end up with the backside of both of the pockets composed with the padded rear of the original backpack.