<p>A first choice letter like this imposes no legal obligation, and the other schools to which you’ve been accepted would not revoke their acceptances even if they knew about the letter (nor would they otherwise hold it against you). As to whether your letter imposes a moral obligation, this is between you and your conscience. I take my word seriously, so if it were me, I would honor the letter. On the other hand, I would never have written the letter in the first place, as I find the practice of “first-choice” letters inherently objectionable.</p>
<p>If there are any third parties involved (e.g., you asked someone to intervene on your behalf with the understanding that if you were admitted you would go), then I definitely think you should feel obliged to honor your commitment. If not, then it’s a balance between your conscience and your belief (assuming you believe) that another school would suit you better.</p>