I believe that once your application is submitted, you cannot change your essays or anything on the common app. However, if you are deferred to regular decision, you can notify all your schools of anything noteworthy that happened since your application was submitted, i.e. major awards.
I would also like to take this opportunity to say that if you are not matched, you may receive e-mails from QB schools asking you to apply ED. Do not discount those e-mails if you are truly interested in the school. I won’t say that it is a shoo-in, but it does mean that the schools are interested.
I worked with someone who wasn’t matched but was asked to apply to Columbia ED via e-mail after the match round, he is currently Columbia Class of 2018.