Based on your history and the time that has passed since your original surgery, it is recommended to schedule a personal consultation with a board-certified surgeon. There are several highly experienced plastic surgeons who specialize in corrective surgeries and can provide you with a tailored recommendation. During your consultation, discuss your needs and lifestyle, which can also help if a full or partial lift or augmentation will help reposition the breasts for a more symmetrical appearance.
If you are having any kind of breast surgery, most surgeons would encourage you to have a 15 year old implant replaced.
With such breast asymmetry as you describe, it might be time to consider a lift again on the sagging breast, and as the implant is now 15 years old, an implant replacement might be in order and help your surgeon with the asymmetry issues fully.
Thanks for your question ashleyfox. The best way to determine your surgical options is through an in-person visit with a board-certified plastic surgeon. We see your situation commonly in the reconstructive population. Based on your description, you are likely either a candidate for some capsule release with a possible implant exchange on the augmented breast vs. a lift on the non-augmented side vs. both. Hope this helps.
Best of Luck,