I was looking at a few tools that allow you to modify the evolutionary trees of Pokemon and it seems that you can actually add in cases to evolve into any Pokemon you want, so degeneration of a Digimon can be done by giving the Digimon a "new" item that would trigger the evolution to the lower stage.
I even found a video of Shaymin evolving into Shaymin. Sadly I cannot link yet, so just google "Shaymin evolves into Shaymin" and it'll be there.
Also, degeneration may not be required. Maybe Digivolution works completely different in the Pokemon world!