I was fortunately accepted to a strong graduate program today. It really is a great program (e.g. top 25 ranking in Physics/Astronomy) but was largely a secondary plan. I will call this option Program B. My main goal has always been to pursue a career in a very different field and I am still waiting to hear back from this program. This program is at a different institution and I will call it Program A. Program B has offered me admission and I have until early March to accept/decline. Program A has a very late application and I won't know whether or not I am admitted/rejected until early June unfortunately. Both programs start in September 2017. I would be happy going into Program B and seeing what happens, but I know Program A is still my absolute first choice if I can get in. Thus, I am wondering: what are the consequences of leaving Program B before actually starting their program despite initially accepting their offer? Program B is at an amazing institution with hundreds of graduate students in the department I was accepted to. I would of course not want to leave any poor impressions on them for perhaps future connections but am also unsure as to if this is really a big deal or not. Thus I am wondering what your thoughts are on the matter. Perhaps important: Program B is only 1-year long while Program A will take 4 years and I could apply again (if I don't get in right now) after I finish Program B.