If it worked the first time, why change it?
Obviously it wouldn't hurt to change it and describe what has happened in your life since last year, especially if it will strengthen the idea that you are so devoted to dentistry that you would apply again.
But on the other hand, why fix it if it aint broke? The bottom line has always been good numbers (DAT & GPA) get you in to dental school. Very seldom does an essay make or break you.
JMHO