Was it Dr. Gagner who told you this, or was he the one who did your original surgery?
Normally when the RNY is done, the stomach is divided into a small pouch and a huge remnant stomach. Did they remove your remnant stomach for some reason? When revising an RNY to a DS, the surgeon puts the pouch and the remnant stomach back together, like they were pre-op---then he does a sleeve gastrectomy on the newly-rebuilt stomach and changes around the small intestine to the DS configuration.
I'm not certain what your surgeon is proposing to do to you that will make you unable to eat fatty foods. We DSers eat a LOT of fat, but only absorb about 20% of it, and we do take a pretty heavy vitamin schedule. On the other hand, the DS has the very best long-term, maintained weight-loss stats, period.