Is 100 Cc Enough to Go Down a Cup Size?

It is certainly feasible to #downsize to smaller implants and also alter the positioning; if possible. However, decreasing is size may also require a Breast #Lift to to #tighten loose skin, better optimize the end result. Any #revision has risks of unhappy outcome or other complications. Also, the right implant for the revision is contingent on shape of body and your desired outcome.

Often the number quoted to lower the cup volume is in the neighborhood of at least 150-200 cc's and sometimes more.

100 cc is not enough of a difference to reduce the breasts by one cup size. WIthout photos, it is difficult to tell whether this would be a worthwhile endeavor for you. See your plastic surgeon in followup. You also haven't indicated what profile implant was used in the procedure and that can make a difference as well.

There is no fixed definition of cup sizes so this is hard to answer. 

Most surgeons estimate 200-225 cc to a cup size.

If you went up two sizes with 360 cc implants, using an implant about half that size will get to up about one size. !00 cc will make a difference, but probably not a full cup size.

Thanks for your question, best wishes.