<p>Nothing lost except a stamp and the time to write a letter saying why you want to go to Williams. In fact, taking the time to write that letter might clarify for you whether this is cold feet or a true hankering for Williams. By the way, Duke is great school. A sophomore with strong grades who can articulate real reasons for preferring Williams to Duke might be a strong transfer candidate.
Write that letter, then see if you want to send it.</p>