The most damage I’ve done to higher aspects of myself was internalizing a truth about magic that I wasn’t able to really harness, forcing my subconscious to accept it as part of reality, so I’m not all that much of an authority on this.
If you started at the highest/deepest levels and tried to change or reprogram things from that point, then work your way forward to your base reality it might be possible. But you should have a firm understanding of what foxes and spirit foxes are like before hand to give yourself the proper context.
People don’t generally want to lose their humanity, so there’s not a lot of research into it. Humans have very different instincts, mental faculties, social behaviors, etc. I know that Thereanthropes think that they have animal souls, but they still are able to function in human society.
There’s also this idea, which would be slower, but I’ve found that trying to alter ancestry can have powerful side effects.