You are correct. I have been in traditional Medicare with a supplement. I wanted to change from one supplement plan to another. (It was cheaper and offered the same benefits, as supplement plans are required to do). I needed “medical underwriting” to switch. Medical underwriting consisted of a non-medical person calling me on the phone, asking 12 questions. I was denied because I was taking one mg of prednisone, which was on his list of medications which led to denial. Couldn’t believe it. My agent said they don’t like unusual conditions??? The next year, I started sooner, pressed my case and changed supplements. I’m sure if I was in an Advantage plan, I would never have gotten into traditional Medicare. Advantage plans can change benefits, list of providers, etc. at any time. Traditional Medicare cannot.