Last year I got into a physics PhD program at university A in the US, and now I'm doing a one-year master's program at university B (which has a much higher ranking) by deferring a year at A. A is asking me to let them know about my final decision by the end of this month so that in case I don't want to come back this year, they still have a chance to recruit another student.
My situation is I actually have submitted applications to my current university B for their PhD program (which has a different focus on physics, and there is no professor at A studies this direction), and the admission result will be available in another 6-8 weeks. I'm honestly not 100% sure which field I really want to pursue at this moment. My question is if I'm telling A I'm coming back, but later at some point I changed my mind, is it unethical if I tell them I want to change my plan and commit to B as their research focus better suits my interest?