It sounds like you were told it was impossible (which you know is not the case) instead of being given a denial. It was a clever way to avoid making a denial, which you could then appeal.
Since you have now had your consult with the bariatric surgeon, you could send him/her an email, very politely stating that you know that a revision of gastric bypass to the DS is possible, that it is done by Dr. Rabkin of Pacific Laparscopy in San Francisco, and requesting authorization for a revision to DS with him. Rest assured you will get a denial, but this puts everything in writing so you have something official in your medical record. You can then start internal appeals, which will eventually get you a final denial, and at that point you can go to external appeals.