I have an 18 replacement hull for a 170, and the differences I found are predominantly in the use of fiberglass and in the design. You should be able to see the obvious design changes by looking at the specs and photos/drawings of the two (length, chine, cockpit design, etc). As far as the rigging and sails, I was able to easily rig the 170 mast, boom, and sails with no problems at all. The 170 sails are a bit smaller than the new 18 sails, but I don't know how much that will affect the performance (for a beginner like myself, I probably won't be able to tell much at first). We'll see when I finally get the boat on the water this summer.
I'm not sure what you would do to retrofit any of the 18 elements to the 17 since the most drastic things are structural and design elements. And some of the stuff in the H170 Owners Mod section will still apply to the 18 - I plan on doing a couple of them to my "new" boat.
Dean