Can I go from a saggy B to a AAA? (Photo)

It is certainly possible to reduce breast size down to an extremely small size, even almost leaving you flat if that is what you desire.  The key is to remove breast tissue and tailor the skin so that the resulting chest contour and skin smoothness appear normal.  This is occasionally done for women with a high risk of breast cancer who desire to lower their risks, and for transgender female-to-male patients.

Thank you for the question. I have had several patients with exactly your goals in regards to the  alleviation of dysphoria and gender nonconforming concerns.
 
 It is possible to reduce the breasts size very significantly. The concern with the amount of tissue removed is related to blood flow to the remaining tissue; if too much tissue is removed in one operation the blood flow to the remaining tissue (including nipple/areola) may be compromised. Part of the tissue that is left in place is called the “pedicle"; this segment of tissue is responsible for delivering the blood supply to the nipple/areola tissue. If the pedicle is made too small (in the effort to reduce the breasts as much as possible) then patient will likely have problems with tissue survival.  
For most patients in your situation, we have been able to decrease the breast size very significantly and achieve the patient's desired outcome with one operation.  Sometimes, a two-stage approach will allow for safe reduction and achievement of the patients' "very small" desired outcome.  Much of course depends on the patient's starting anatomy and carefully communicated goals.
My best suggestion: seek consultation with board-certified plastic surgeons who can demonstrate significant experience achieving the types of outcomes you will be pleased with. This careful selection of plastic surgeon will be the most important decision you make.
Before undergoing the breast reduction procedure it will be very important to communicate your size goals with your surgeon. Most patients wish to achieve a enough of a reduction to help with their symptoms while remaining proportionate with the remainder of their torso. I suggest that you do not communicate your goals in terms of achieving a specific cup size. For example, a “A cup” may mean different things to different people and therefore may be a source of miscommunication. In my practice, I ask patients to communicate their goals with the help of goal photographs.
I hope this, and the attached link/video, helps. Best wishes.

A breast lift can be done even if going from a small B to an A cup.  The technique can be modified to reduce visible scars, opting for a vertical lift instead of a Wise pattern or inverted T type lift.