Losing weight after breast augmentation with fat transfer - will it impact my breasts?

Fat transfer is sensitive to changes in weight. Transferred fat cells are vulnerable to weight loss and can shrink in response. The effect is loss of volume in grafted areas.

As with any procedure. the best time to consider surgery is when a patient has reached a healthy (BMI less than 30) stable weight. This ensures a lower risk of complications as well as longevity of result.

With regards to your specific question, it is impossible to predict the results of weight loss with any certainty. Weight loss of 5-7 lbs. is unlikely to make a significant difference in result. 10-15 lbs. is more likely to be noticeable. There is no guarantee breast volume will remain stable while total body fat is otherwise reduced.

As always, discuss your concerns with a board certified plastic surgeon (ABPS).

Since your implants are under the muscle, you are less likely to see/feel rippling compared to if the implants were on top of the muscle. Other factors to consider are how much native breast tissue you have, how much fat was transferred to the breasts, and where exactly in the breast the fat was transferred. Based on the information you provided, your BMI is 20.8. Since you started at a size A before your augmentation, I would venture that you had only a modest amount of native breast volume before surgery.
Taking everything into consideration, with 10-15 lbs weight loss and implants being under the muscle, I wouldn't worry too much about developing a "deflated" appearance or significant rippling. With the weight loss, you might notice a slight decrease in the size of the breasts from lose of volume from your native breast tissue and transferred fat, but it shouldn't make a drastic difference. With that amount of weight loss, your breasts might "appear" larger when you compare them to your current body proportions, but since the majority of your volume comes from the 350cc implant, weight loss could make the actual total breast volume (implant + native breast tissue + transferred fat) decrease.