I had the DS, and I did not lose my disability, since my issues are systemic. I have Lupus and Fibromyalgia. SMO was listed as a disability initially, but the systemic issues don't go away. If SMO is your only disability, I'm guessing weight loss could affect that. If you have obesity-related co-morbidities, and they go away after surgery, that could well affect it too, unless there is something else that keeps you from working. I still have flares, but I haven't had one that put me in the hospital since my surgery.
Social Security has a great program called "Ticket To Work", where you keep your full benefits during a nine month trial work period. If that's successful, then your benefits are reduced to make your income equal to what you were getting on full disability. If you make more than you're getting after your TWP, then your benefits go away, but if your disability flares up again, iti s easier to get full benefits reinstated.
There's lots more information on www.ssa.gov. I know about this, because I'm participating in the Ticket To Work program.
It was my intention to get this surgery so I could get off disability, and get back to being productive. The other health issues I have are more manageable since the weight loss.