Switching fields after MA
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.