Hi, Barbara,
As is so often said, but no less true, everybody's different.
Your question is best put to your surgeon and/or your pcp. And I would ask sooner, rather than later, just so you won't have to be concerned about it.
My son had the MGB 8/10/12. His surgeon took him off diabetes (metformin) and bp meds the night before surgery; he has not gone back on them. His bp readings are excellent; blood sugars consistently in the 90s.
Diuretics in general and Lasix in particular are a whole other issue. I can't imagine your doctor will continue you on Lasix post-op. One of the characteristics of Lasix is that it WILL take fluids from your body, regardless of whether you have any extra to give. And post-op you will need to be diligent to take in sufficient fluid to keep you healthy; you certainly are unlikely to have any extra.
Congratulations on your upcoming surgery! You must be very excited!!!