How can I handle the social consequences of a breast reduction?

Diplomatically. Breast reduction surgery is almost always done for medical reasons. All medical problems carry their own social stigmata. You should simply inform your friend and family that the size of your breasts is causing you health problems, and you will feel better after surgery.
On the upside. The social consequences depend on you. They may be all good if you gain improved self confidence and are able to walk without pain or discomfort. I commonly observe hunched over patients who can now walk upright with their shoulders back exhibiting improved posture. Patients are often less inhibited about concealing their chest and clothing restrictions and often undertake new exercise activities.
No Problem. Generally speaking, breast reduction surgery is one of the most patient pleasing operations plastic surgeons perform. There are no specific “social consequences” that I am aware of. People who care about you will likely be very happy that you will likely experience improvement/relief from symptoms caused by the disproportionately large breasts. Best wishes.
I'm stumped! I am sure this is a good question but i really can't figure out what you mean. I will guess that you are asking how to handle questions from "friends" who notice that your breasts are smaller after surgery? Just tell them that you had a reduction to help your back pain. Again, please give me a little more info when you ask again!
If you are concerned. About anyone out anyone other than family you should question what value that relationship has. Inappropriate behavior is to be dealt with the usual ways.
By Celebrating! You will look slimmer and better in your clohes! you will feel better with relief of neck and/or back pain! you will be able to more easily exercise, thereby losing more weight, looking and feeling even better! so simply smile demurely, be gracious when complimented, and live well!
Consequences? What social consequences? I assume the reason you are having a breast reduction would be to relieve back and neck pain, fit more comfortably in clothes, or maybe you are self conscious about having larger breasts. Whatever the reason for having this type of surgery must be a personal decision. Who cares what other people think, it is your body and you deserve to feel comfortable.