I love this thread. I know it's ancient but I feel it's really important so I'll put my experience in too.
I was sleeved 13 days ago. I am still nursing my 16 month old daughter perfectly well. I nurse 4-5x a day and she has had no complaints. I asked the staff at my clinic if I could continue to breastfeed during all my preop appts and they said no. That I "couldn't" while I was on the 2 week preop diet because it wouldn't be safe for baby. (It's just low calorie, high Protein food. Nothing unsafe there.)
They also said I couldn't because of the anesthesia. (I wasn't gonna nurse in the hospital anyways. In fact, I didn't even pump. I didn't get engorged until nearly 24 hrs after my surgery. I tried to hand express but I gave up quickly. My boobs don't like giving up milk for anything but baby.) I was released from the hospital 36 hrs after surgery and went straight to my baby and nursed perfectly fine.
They also said that I would get so few calories after surgery that I "wouldn't" be able to nurse. I have averaged 500 calories a day and she hasn't complained yet.
Don't wean unless you want to.