The consensus I've seen around SDN is that if you wrote a letter of intent to one school, it means that you've basically said that no matter what happens, if you are accepted to school X, you will attend that school despite having other offers from Y, Z, etc. It looks really bad on you if you get accepted to school X, but decide to go to another program that you were also accepted to. Why would you have given a letter of intent so early before March? Clearly, it wasn't a good idea since you've clearly changed your mind since then.
From my phone calls with ARCOM, I was told that they don't place any serious emphasis on LOI. The reason for that is basically this post right here. Too often students use letters of intent in an attempt to convince ADCOM members to choose their application, but don't mean the words they say because as soon as a better option comes around, that letter of intent and the promise that student made to attend that once beloved school becomes meaningless. My advice, don't bother wasting your energy on making another letter of intent cause its not only disingenuous, dishonest, and unethical (according to some individuals on here and in some medical school admissions committees), but it won't place help your case in any way whatsoever.