I know it may not seem like it but you still have quite a few options open. Have you taken seminars on medieval art while at the university? If it really does appeal more to you and you’ve developed a good relationship with a professor you trust who specializes in medieval art, you can talk to them about the possibility of studying under their advisement instead due to a change in interest. My advisor switched from a Renaissance specialization to a Baroque specialization while completing her PhD program. Her change is more subtle than yours, but think about the possibility of studying under a different professor. Changing specializations doesn’t mean you need to drop out of the program completely.
There’s also a PhD student at the University of Chicago who specializes in African art but is doing her dissertation on the cultural and material exchanges between Italy and countries in Africa during the Renaissance. Perhaps you can combine both of your interests in a similar manner.