The most noticeable difference is the "S," which signifies a seven-inch height instead of eight. This reduces weight and allows uniform trousers to lay better for a more professional look without sacrificing support.
Other differences include adding a flame retardant tongue and outer shaft; this too reduces weight and improves the breathability of the boot. If you have the original boot, you will notice the locking lace loops have been replaced with cloth loops, one lace lock on the ankle, and two traditional eye loops; this makes the boots easier to lace and unlace. The interior arch wrap is made of abrasion-resistant leather to protect the boots when fast roping, a feature found on very few duty boots. The last exterior change is the toe cap which is now one piece instead of a wrap. This al lows you to get a high polish on the boot.
For first responders the most significant change to the boot may be the moisture barrier. Instead of the original GSG9's Gore-Tex, the GSG9S uses Crosstech, which is bloodborne pathogen-proof as well as waterproof. This is an important safety upgrade to greatly reduce the odds of your being exposed to any blood- or liquidborne pathogen.
One thing Haix didn't need to upgrade was the fit and comfort of the GSG9S. It retains the Haix MSL System, a special heel spring and toe to make your gait easier and more comfortable. And the boot's PU nonmarking sole is conveniently oil-, chemical-, and slip resistant. Both the MSL System and the PU outsole also insulate you against the heat and cold.
With all these changes I had to see if the "S" was as good as the original. I soaked a boot for a couple of hours in the wash tub and the tissue inside remained dry. By virtue of my highly "scientific" study I deemed the boot waterproof. Still, I hope not to be standing in several inches of water for hours at a time.