There is no downside to rewriting your personal statement, but there is a lot of upside. I would rewrite completely since it gives you the opportunity to think about why medicine is a good fit for you. I didn't have huge changes between my motivations between last year and this year, but I did sit down blind and rewrite my personal statement telling the stories in a fresh way. I was pretty successful this year, four interviews, three acceptances, one waitlist on my fifth cycle. Were there a lot of fundamental changes in my application? No, but I did what I could to make it fresh for ADCOMs, and all three schools that accepted me hadn't even interviewed me before, despite being a reapplicant. I haven't sat down and compared my personal statements, but I know I used some of the same material, but I used different words and brought in anything new I thought was relevant.
