After being with many people who have lost weight it seems to me if you're meant to have them you keep them and if you weren't, you don't.
The second place I gain and the first place I loss are my breasts. So I go from an almost D to a B. Or rather I return to being a B as the B is my natural state in life.
One of the ladies I originally lost with lost almost 100 pounds and kept her Ds - that was just her natural size.
I remember back when Jorge Cruise's 8 minutes in the Morning first got popular and he believed that women want to lose so they jump on the treadmill and stop eating properly and that aggravates breast fat loss. But if you're going to lose, you're probably going to lose.
Continuing with weights is a good idea because while they can't enhance your breast tissue/fat they do build up the pec muscles they sit on. I know a couple women that did lose a fair amount of fat from the breasts but since they also became lifters the breast they did have was quite perky and upright (XD) from the strong muscle underneath lifting the breast slightly.
Chest exercises! (which you probably already do)
Chest fly. Chest flys are one of my favorites because they work the side of the chest and are going to help keep your breasts in line. You can also do flys with cables or a machine in addition to the dumbbells pictured here.
Chest Press. Probably the weight exercise we all learn first, it targets the large chest muscle and strong chest muscles will assist in keeping your breasts perky. Can do with a barbell or a machine, as well.
Isometric exercises like clasping your hands in front of your chest, with your elbows pointing out to the side, and pressing your hands together. Or the famous "we must! we must! we must increase our bust!" exercise from "Are you there God? It's me, Margaret." where you clench your fists and thrust your elbows back and forth in a pumping motion. Which, sorry Margaret, doesn't make your breasts bigger but the shef they sit on.
xd Since you can't spot reduce and will have to do cardio if you want to lose body fat the breasts will always be at risk and even with lifting to keep everything up and out pretty sure you either lose or you don't. The only reason I can think of that would for sure lose would be if you lost a ton of body fat and it would have to start coming off everything. That's the reason female figure competitors and body builders rarely have much breast, it's not that they lift too much weight but that they've continued a fat lose program that brings them well down from what even a fit woman would be at so their muscles will "pop".
I mean, I want to try for around 20% body fat but a figure competitor would be around 10 or lower. For women, our essential body fat is 10-12 percent so that's pretty scary.