insurance PPO to HMO?

So when I had my surgery done I was under a PPO plan, now my insurance has switched to an HMO and according to my dr.s office I have to pay for the office visit until I get pre-approved by my new insurance for them to cover it. How do I go about doing this, I'm worried that they will say that they won't cover it because it was a pre-exisiting condition and I can't afford to pay $200 every time I need to go to the doctor. If anyone has any info on this please let me know, thanks.