You might think you'd go down to a C, but you never really know how your body will respond to weight loss.
I figured I would go down to a C (I still might, who knows?) but I went from a 40D to a 36DD. My bust measurement went from 44" to 41". So while it decreased, I still went up a cup size because my BAND SIZE decreased. I haven't noticed anything different about my breasts yet (except for the fact that my old bras are too big).
You and I have youth on our side though

It's much easier for a twenty-something's body to "recover" after weight loss than it is for someone who is much older.