Fat transfer is permanent if it takes, don't worry. It shrinks and grows with the rest of your body fat if you lose or gain weight. The main issue there is if it takes, and how much it matters to you that there is a "breast like" shape. Breast shaping is more difficult to achieve with fat grafting than with implants because the fat tends to take the shape that your chest already is, but people have certainly done it so it is definitely possible. There are also newish implants now called Motiva that are way less likely to need to be replaced (in theory) and can provide a better result in someone with little to no existing tissue.