Ok, so I lost 3 stone (from 12 to 9 stone) over 18 months and my boobs went from a 38 D to now a 32 A. Your boobs go before your stomach, most girls find i think! Now, I've never been concerned with them getting smaller - like yourself - but was also worried about the inevitable sag. And, mate, it is inevitable, I'm afraid. They're not terrible, but they are a bit loose. And i'm still rly young too, so I get the 'young girl - saggy boobs' terror too! However, you gotta think, you're gonna feel so good about other bits of your body once you lose that poundage, and also, there are so many great bras these days that you do not need to worry about it AT ALL unless you're naked. Seriously, once I put these sagsters in a bra, you cannot tell at all that they're not perky.
So just keep at the weight loss and focus on how great you'll look once the pounds start sliding off! Have fun selecting new and wonderful bras for your new and wonderful body