Your situation isn't simple. You are (if I understand things correctly) looking not just for a DS, but for revision from RNY to DS. There are only a few surgeons who do this very complex surgery.
While Dr. Ferrari does have the DS listed on his website as something that he does, I have never yet encountered a single DS patient of his on this or any other DS related website, and I've been here over 7 years. So I'm not saying he's never done one, just that he probably hasn't done many and doesn't do the DS regularly, and may never have done a RNY to DS revision ever.
With the passing of Dr. Stewart, your best bet in TX is probably Dr. Ayoola, who worked with Dr. Stewart until his sudden and unexpected death, and who is now continuing the practice. I don't know whether or not he accepts Medicare, but it would be simple enough for you to ask. But the biggest question is not who accepts Medicare, but rather, who is qualified and experienced in the specific operation you are seeking, which is not a straight forward, virgin DS. You could find 100 surgeons who accept Medicare but if they don't do the right operation, that does you no good.