<p>If you no longer want a major within the CoED, you'll have to change colleges, which is done on a competitive basis.</p>
<p>You don't need to do it before you send your SIR. You'll need to do it afterwards, when you actually begin taking classes. For the most part, you'll be admitted or not based on the grades you have in CoED, which if you're transfering to Letters and Sciences, shouldn't be too hard.</p>