I agree with JP. I'm a podiatrist and I know that I'll be getting that question if and when I interview. Never disparage your profession. Don't say you want to be a DO because of a negative reason about being a PA. I just said that I wanted a more complete education and wanted to be a "whole body" doctor.
My personal statement emphasized my background, but I was sure to point out that learning never stops and whatever I did before that may be repeated will only be bolstered. I, of course, also spoke of what brought me to osteopathic medicine and how comfortable I feel with the tenets and underpinnings, etc.