Can I go from a 32DD to a 32B?

First you need to wait until you have stopped breast feeding for 3 to 6 months.  After you have stopped producing milk you can reassess your size and goals.

You can certainly diminish your breast size but you must stop breast feeding for 6 months to allow your breasts to 'dry up' and shrink so that a more accurate assessment can be made.  As for going down to a B cup, its difficult to predict whether a B will be accomplished but a reduction certainly.  Sometimes revisions may be needed to help you achieve the goal you want but you may better appreciate that a C cup is good once you get to live the life with it.

Congratulations on your decision to proceed with breast reduction surgery; it is one of the most patient pleasing operations we perform.  Ultimately, in-person consultation with board-certified plastic surgeons will be your next best step.
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. prior to in to undergoing any further surgery, it will be important to communicate her goals carefully.Yes, 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.  
The other concern with overly aggressive breast reduction surgery is patient dissatisfaction  afterwards.  It is not unusual for patients who have lived with very large breasts to want to have as much as possible removed. Care must be taken to be judicious in this removal to avoid an outcome where the breasts  are too small in relation (proportionately) to the patient's other body parts.  Again, it is not uncommon, for patients'  breasts to become smaller ( after the breast reduction procedure) with time and/or weight loss-  breast augmentation may become necessary to achieve the patient size goals.
I hope this, and the attached link, helps.

You can decrease your beast size through a breast reduction. However you first need to wait 6 months after breast feeding to allow the breast to dry up and normalise. At that point you should seek a Board Certified Plastic Surgery to help you obtain your goals.Good LuckStephen M. Davis, MD, FACSGreen Hills Plastic Surgery

The breastfeeding is very important for your baby at lees the first 6 months, after that you can stop the breastfeeding, and when you don´t have milk production it would be the good momment to considered a plastic surgery of your breast, in this momment it is very probably that your breast has change and return to the real size and it is probably that the breast are more falls, for any change is the best momment for the surgery.