Switching fields after MA I am a Ph.D. student at a top 15 program in my current social science field, I'll call it field X. My research is interdisciplinary standing at the intersection of field X and another field Y. 

I genuinely like graduate school and my work here, but as I've developed as a scholar, it has become apparent that my true passion (and resultant new research questions) lies clearly in field Y. How frowned upon would it be to master out of my program in field X and try to move to field Y? I've heard mastering out of a Ph.D. program is a huge red flag when reapplying, and I think I'd enjoy a future in field X more than any other job save for a future in field Y. I also feel very indebted to my current program and advisors as a result of the resources they've invested in me. 

Would it be advisable to give up on a future in field Y and remain in field X? Or would moving not be so terrible since it's due to a genuine (slight) change in research interests rather than not liking graduate school, or clashing with advisors, or anything like that?
I doubt that you have any need to move and disrupt your trajectory. If your research is truly on the cusp, then you can probably move seamlessly into field Y even with a degree in field X. People change fields fairly regularly, especially if the change isn't radical. All of my education was mathematics, but lack of job opportunities when I finished pushed me to computer science where I had a long career. 

The only problem, I think, would be in finding your first position. You need to have some contacts in field Y, perhaps professors, who will be happy to support your entry into the field, regardless of your doctorate. 

And there is great value in interdisciplinary work in any case. So, building a circle of collaboration with people in both fields would probably be an advantage to you in the long run. 

Once you are employed in some permanent position (tenured or similar) your choice of research is pretty much up to you. No one will care, anymore, that your degree is in a related field. 

Having a degree in field X needn't lock you in to working only in that field.