Gastric bypass is not typically reversed as the new stomach formed will eventually stretch to accommodate more food (like the old stomach did).
Having any procedure reversed involves the same risks and complications of any surgery, which are the potential for blood clots, post-operative bleeding, infection, and pain.
Considering reversing gastric bypass also requires consideration of the changes in the digestive tract, which means potential complications at the new site of anastamosis (or surgical joining) as well as leaking of the stomach pouch and potential for ileus (or the bowel staying asleep after anesthesia and requires additional surgery).
Reversing gastric bypass offers the benefit of no longer needing daily vitamins and improved digestion with fat absorption, but if you are considering gastric bypass, you should discuss with your surgeon potential side effects and the options for reversal, but buyer beware, having the surgery is a complicated decision not to be taken lightly. Having it reversed more so.