I was waitlisted by my first choice school and accepted to my safety school. I accepted my safety school's offer on the 15th, and today (17th) they sent me the GRA contract. I have not signed it yet, since I wanted to check with my first choice to confirm that there was not a slot for me. However, after everyone had ended up choosing where they wanted to go, the internal fellowship I was waitlisted for at my first choice had open slots. Since they got more funding, I've been accepted to my first choice. I haven't signed anything for the safety school or put down any deposit, just sent an email saying I'm attending. How do I go about telling my safety school that I'm no longer attending? Do I just thank them and vaguely refer to changing plans? I'd like to do this as politely as possible.