I kept the core of the essay essentially the same and changed/updated anything that needed to be or that I thought didn't sound too great. Basically, my essay begins with a story about a very defining experience in my life that greatly influenced my desire to become a physician...a year later this experience is, of course, still the reason I am pursuing medicine. Therefore, I kept that the same (but cleaned it up a bit, revisions can ALWAYS be made). At my interviews this year, I received a lot of positive feedback on my essay and the experience I wrote about, since I am applying to a couple of the same schools, I thought totally changing that might look a bit strange.
However, in the past year I have gone much further in both my research and clinical experiences so I highlighted this in the new essay. Basically, upon reading last year's essay I noticed a lot of "fluff" pertaining to my experiences and how they make me feel about medicine. This time around I have a lot more concrete examples and info to share, so I feel this portion of my essay is much better. I am a better candidate than I was last year and I think this is conveyed through this years essay, especially when compared to last years.
If you are re-applying to any schools (which you should if you received interviews from them last year), just make sure you do not contradict anything you said in last year's essay.
Good luck.