Emailing potential PhD advisors (recent change in research interest)
Some of the other students in my lab came from different fields. They always showed an interest in what we were working on even if they didn't have the technical knowledge just yet. I think if you let your potential advisor know that you are interested in their area of research, that you have some skills that they are looking for or can be just as useful in the new area, then you should be fine. Just let them know your desire to work in the field and the skills that you can bring to the lab. I don't think its necessary to specifically talk about why you are not interested in field X, but it is important to show that you can contribute to field Y. Do your homework on the lab and talk about specific projects that they have worked on. You don't need to be an expert, but if you ask intelligent questions about their projects, they tend to take that as a sign of interest. Also be ready to relate what you have done in field X to what they are doing in field Y. Often, students coming in from other fields provide a different prospective that ultimately makes the lab better and the research being produced stronger. Good luck.