Going from 36C back to AA: How much sagging should I expect? Can I expect to be completely flat again?

Hi Powerful,
There are many options for you, so the answer really depends on you. At your young age, unless you have had tremendous weight changes, I would generally expect the skin to spring back fairly well. Just removing the implant and leaving the capsule behind will likely give you the look of a very small implant as your body's natural fluid fills the remaining pocket. You certainly have natural fluid collections elsewhere, like in all your joints and your eyes, so that is one option to avoid breast lift incisions and implant issues. If you have a saline implant, you can ask to have that deflated in the office to get a sense of that look before surgery.
Breasts have a trajectory over your lifespan; they tend to be larger every decade or so until about 60. You will likely be slightly larger than you were when they were placed, and the skin will certainly be looser. If after explant, the skin is not tight enough, then adding breast lift incisions can remove the extra skin, even down to a flat, ballet look if you want. That can be done at the same operation, or later on, after a spring-back period to see how your skin behaves. Find a surgeon who can assess your skin, talk through the pros and cons of those options, and make a plan that is right for you.
Hope that helps!

Unless you have gained weight or have had hormonal changes, it is likely that your volume will return to your preoperative state following explantation. Most women your age will not require a mastopexy or breast lift.

Having implants does not increase, but rather decrease your breast tissue since they do stretch and thin your tissue. If you had little prior to surgery, you will still have little, unless you have gained weight. You will have some loose skin and may need a lift, or may can have minimally invasive radiofrequency tightening. You might consider going down in size and tightening your pockets internally. Remember, you were not happy before getting implants and may not be happy once removed. Going down to a B cup may be the best solution.