I have not seen one and I'm not sure a parts schematic for the 590S has been released yet. But here's a write up that explains the three major changes.
If it ain’t broke, don’t fix it. Mossberg appears to have agreed with this principle of design while reworking the 590, as most of its well-proven design remained the same in the new Mossberg 590S. The only “problem” worth addressing was the design’s inability to reliably feed mini shells—something that virtually no pump-action or automatic shotgun has been able to do consistently. To remedy this, all that needed changing on the 590S were its elevator and bolt slide. An energy-absorbing buffer was also added. The end result is a shotgun that feeds 2.75 and 3-inch shells just as reliably as before, but now with the ability to add 1.75-inch shells to the mix. Mossberg claims that the 590S will run reliably with a mix of ammo in any order, and varying load types or shell lengths will not cause problems. If it’s a 12-gauge round it can be stuffed in the tube, pumped into the chamber and fired without issue.
Hopefully someone can do a side by side breakdown of the two models to show the design differences with photos.
Regards