Interesting technique, I've toyed with the idea of swappable buttons but never gotten this far. A buttonhole stitch (where you are "knotting" the thread with each stitch) might work even better. I also wonder if a small metal washer would take the place of plastic ring, and give the button better support. But you'd have to make sure the edges were smooth enough to not abrade the thread. All in all a great tutorial, thanks!