NWA will not be replacing the DC9s completely with the 175. Perhaps you'll see a few routes right sized, but overall the 175s will not come close to providing the proper replacement. The good news, IMO, is that the 195s could very well be a reality, only they'll be flown at mainline, where they ought to be flown.
Can't wait to try out the 175 myself... and the 900 too.
AZJ