Will 515cc implants sag in a couple of years? (Photo)

It is possible but it's best to wait and see. It's difficult to predict per your photographs and not seeing the condition of your breast tissue. If you do experience #ptosis or sagging in the near future, a  Breast Lift (#Mastopexy) may be considered.  A Breast Lift can  be done while having existing Breast Implants and can also  be used to correct #asymmetry if necessary. In the meantime, remain  patient.  Ask questions of your surgeon to discuss any of your concerns. 

Thank you for the question but  it is a somewhat confusing one. Undergoing revisionary breast surgery, with breast implant exchange, because of an “allergic reaction” does not make a lot of sense to me. Your plan makes sense if you wish to achieve a larger breast augmentation outcome;  whether or not you will be pleased with the long-term outcome of the procedure cannot be determined online.  Your plastic surgeon, after  careful evaluation of your body type, careful measurements (dimensional planning), and a full communication of your goals will be your best resource.  Unfortunately, poor scarring remains a risk, despite best efforts. I hope this,  and the attached link (dedicated to revisionary breast surgery concerns) helps.

Hard to believe via the internet you are just a scar former vs allergic to sutures??? Best t sep other in person opinions...