Here's an example from real life. One of my undergrad English professors had all his degrees in English. But somewhere along the line, he became very interested in the very earliest writings, like Babylonian, and started studying those. That led to a PhD in history--he didn't need to go back and take an undergrad history degree. He found a university that would admit him to their grad history program based on his existing degrees and the independent studies that he had undertaken.
I remember this because the semester I need him to write recommendations to grad school for me, he was away on an archeological dig in Turkey, which in the days before email and the internet, meant that getting recommendations from him was not an easy thing.