Accepted a job in one area but a new passion has developed

Let me preface by saying that I realize how fortunate I am to have either the OR or ICU as an option for my first nursing job. Also, I'm valedictorian of my class, taught myself about critical care-hemodynamics, vents, EKGs, etc.-since our program has no critical care class, and have confidence in my ability to succeed in either of these disciplines with the proper mentoring. I probaby would've gone to med school if I had been a little more focused in college, but as my dilemma illustrates, focus is not one of my strengths.

The short version: my program has no OR or ICU clinical experience so I set up my own OR observations, watched surgeries, joined the AORN, read books and decided OR might be right for me. I was offered a job at the hospital affiliated with my nursing school in September (I graduate in December) and I verbally accepted in early October. Fast forward to our 200 hour senior synthesis when OR was not an option. I took a position in the surgical ICU figuring it would be relevant. 36 hours in, I am in love with the SICU and now seriously regretting my decision to work in the OR.

I originally picked OR because I like the specialty, like the organization and order, like the possibility of RNFA and thought it would make me marketable for travel nursing/medical missions.

I like the SICU because the cases are varied, I'm learning a ton of skills, there's a huge amount of autonomy and clinical decision-making, I like the possibility of working nights and I love the atmosphere. I'm not sure if I could get on at a place like Doctors Without Borders with ICU experience since most of the developing world doesn't have vents and LVADS. Also, I am planning down the line to get an acute care NP degree so either area would be relevant.

I feel like a made a commitment to the OR so I should stick with it, but I'm not sure if it is viewed as a commitment by HR since I haven't signed a contract. Plus, two years feels like such a long time (The ICU commitment is 1 year.) But as both opportunities are at the same (only) major teaching institute in town, I don't want to talk to an HR person to be told that it would be poor form to back out and then start my job with a negative impression of me...