Our first year was internship (2-4 program), and I didn't decide to change until about 2 months into my R3 year (2nd year of EM), and I decided to finish out the year as to not completely screw all my classmates.
I think one reason it took me so long was that I LOVED my program, all of the residents were really close, really liked all of our faculty, and I consider our program director to be one of the most amazing physicians and mentors I have ever worked with. But I just HATED the job. It was also really hard to admit that I had not made the right choice -- I felt like such a loser.
But it all worked out well, I felt like I did the right thing by finishing out the year, and my EM training has certainly made the first few months of anesthesia a lot easier.