Keeping strap length the same with be crucial to fit, so keep that in mind.
The cleanest and most labor intensive option will be to take apart the arcing shoulder seam on both packs, add the new straps, and sew it back together. Removing the bar tacks with a seam ripper is a nuisance, and you'll want to position the new straps just right, but the hardest part might be that the old stitching on the Jam will leave the fabric pretty perforated, and new stitching could weaken it. I've not taken that seam apart myself, so I'm not sure how much of a seam allowance is inside the binding. This could in short prove to be a delicate task.
The simplest way would be to cut straps off the Peak as close to the pack as possible, and cut the Jam straps off you have a good 4" left attached. Peal the outside fabric of the Peak straps back and cut an inch or two of the foam off. Do the same with the Jam straps. Do a felled seam to join the straps, with the foam parts as close together as you can manage. Then fold the fell back over the Jam straps, and sew a box stitch through the Jam straps, including the foam. The thinner foam on the 2012 Jam straps should be thin enough to get through with a standard machine, if you crank the thread tension and go slow. Be super careful to get the alingment dead on, and the overall length close to the original.
You'll obviously have to lose the load lifters with the latter method.
I'd probably go with method two. Less chance of weakening the underlying construction, and faster.