So here is my experience, and my husband's, and my son's (we all have UC). The 100% covered colonoscopy is for routine preventative care (and at the same frequency, like every 5-10 years). Once the procedure is for monitoring a known condition or for diagnosing symptoms, it is not routine preventative and is not coded as such, even though there is a preventative component.
That said, at least my premium rates are not based on my diagnoses, but only age and zip code (at least for now).
Jan