If you have an example, for instance, of how you first got interested in medicine, keep it. Try and either use new examples or completely reword old ones to fit a new theme that you can think of for the rest of your personal statement. Make sense? Trust me when I say that I used to think that I couldn't change so and so example. Yes you can - just brainstorm and then brainstorm some more. Every story/example can be looked at from multiple angles.