Short answer......no, it will not ruin the work that was done. I presume you are having a flap reconstruction, if so, the size of the flaps will tend to follow the rest of your body with respect to weight changes. I like to set the body shape with liposuction at the second stage to allow for much of what needs to happen with weight/body shape changes to happen under my surgical control. Then the added fitness and weight loss you pursue will generally not require as much overall body weight loss to get to where you want to be. This tends to help preserve the size of the new breasts or at least leaving them larger than the rest of the body in a relative sense once the weight changes happen.
Long winded......but rest assured, your surgeon's artistic eye (assuming they have one!) should be able to project what needs to happen now to effect a quality and lasting result for you.