DVD Player full screened already upconverts the video... that's all an upconverter is doing, taking a lower resolution video and fullscreening it, with a little bit of blurring to smooth out the pixels.
If you rip a DVD and view it at full screen, you're upconverting.
Yes, many people can't tell the difference between upconverted content and HD content. It's not that upconverted content looks amazing, it's that people can't notice the improvement.
On video games, where lower res content has jaggies, HD make a significant difference in visuals, but 480p video doesn't have jaggies...