Diabetes management is essentially a balance of energy in (mostly carbs, some fat and protein) and the stuff you do to bring it down (exercise and drugs for example). If you reduce the energy going in by halving your stomach's capacity and keep the same doses of drugs and the same amount of energy expenditure through exercise then your blood sugar goes down and you become hypoglycaemic. This is just as bad as your sugars going high (hyperglycaemia)
Obviously, this is a massive oversimplification but you get the gist
Managing the change is not always easy. You get it sorted in the end but expect a bit of faff while it all gets sorted. The CAA may want you not to be solo pilot while it is