Always re-write! Even if the core message is the same, re-word it! After I got rejected from UWash last year, I had a chance to meet with the Dean of Admissions to talk about my app, and as she was going down the list of elements, she told me, and I kid you not, "Your personal statement is excellent, however, you have to re-write everything, if you don't we won't even look at you, it will look like you didn't care enough to give it the time it deserves. In the case of your personal statement, I'd love to tell you not to change it, but since I can't, I suggest keeping all of the same elements and just re-working it." While she did say this specifically about my PS, I got the impression that it extended to all essays in your file. Obviously the "we won't even look at you" policy will probably vary from school to school, but why chance it?