I posed the same question a month or two ago here! I had applied 3 years ago and I really liked my PS (and I got a lot of positive feedback from others). In fact, I got numerous interview invites at "reach" schools and I'm willing to say that part of that was probably my PS... Also, when I read old essays, I usually grimace at the immature things I had said, but not with my PS.
I'm still not sure if adcoms really look at your old PS or not... Some say "no," they have enough reading to do as it is; Others say "yes," and they want to see that you've "significantly improved" since last time, and that you've at least put in the minimal effort of re-working your PS!
For me, I took my old essay and decided to just add in a new paragraph about what I've been doing in the past year. This required me to cut out some old material... and somehow in the process, most of my most material got cut off and replaced with new material. My motivation or my main "theme" didn't change, and my intro/concl was essentially the same... but between drastically different way of stating things (more powerful and "visual" words), re-ordering some content, and 1 paragraph of brand new content, my PS does sound fairly different from the last version... Maybe you'll experience something similar? I meant to sit down to "just add in" 1 paragraph, but my PS did go through a make-over after all...