I guess technically if your PGY1 year and the successful completion of it is not required for your new training program, then yes, you could leave.
However, as noted above, why burn bridges? Medicine is a small field. We had a Prelim resident who did the same - basically got a position in the scramble and called and said she would not be coming in again. Not only did that upset everyone (adding to their call schedule), it was unprofessional...our PD made sure her new PD knew about what he was getting.
Lee: Bit-o-trivia -- when they were writing the pilot for Scrubs, the writers posted on SDN looking for funny stories. There's the belief that "Dr. Cox" is named after our own "Dr. Kimberli Cox".