Can I get a cosmetic breast reduction on my already small breasts?

It is certainly possible to remove part or all of your remaining breast tissue to give you the size that you want.  Depending on the amount of sagging that you might have, you might also require some sort of breast lift (mastopexy).  I would suggest that you schedule an appointment with a board-certified plastic surgeon who can examine you in person and make recommendations.

Yes, it will likely be possible to achieve your goals. However, I would suggest that you CAREFULLY weigh the pros/cons associated with this type of surgery.  Make sure that the trade-offs, such as scarring, are worth the  advantages of smaller breasts. If you choose to proceed, select your plastic surgeon carefully;  careful communication of goals will also be key (computer imaging and/or goal photographs may be very helpful). Best wishes.

It might be quite difficult to reduce your breasts to a very small size such as an AA without potentially compromising blood supply to the nipple or shape.