Check with the school, as things vary quite a bit from place-to-place. I doubt that you'd have to reapply. There aren't all that many DO, PhD students and many of the schools are eager to have more. Maybe they can work out something for you.
Most of the programs are set up where you do the first 2 years of the DO program (you may have some extra PhD work to do in the med classes), do the PhD, and then go back and finish the last 2 years of the DO program.