Quack, the Gen 1 packs and frames usually had a thinner lumbar pad (and will probably have a very thin one by now). Our sizing has stayed the same but our lumbar pads have gotten thicker which will make the belts fit a little tighter. If you usually wear a medium and have found a Gen1 frame, it will probably fit like a large belt on you. The frame heights will be the same. The Gen1 Bags used a sleeve system that goes over frame and became part of the torso padding. This feature will make using the Gen 1 bags harder to do with our current frames but can be done with modification. Here's our catalog from 2012 that will give you the run down on most of the tactical gear but it looks like most of it is Gen 2.