Would taking out silicone implants to start breast fat transfer be a good idea?

Fat that is transferred is not place in the space where the implant is, but actually in breast tissue. If you are an A cup, you will not have enough tissue for 300 cc's of fat to be transferred. With a transfer, you can expect to gain one half to one cup in size. With minimal breast tissue, you will do better with implants. You can have fat added to your breasts and keep you implants for volume.

Breast fat transfer is a great tool to balance breasts, improve symmetry, and gain up to one cup size in volume.  In general, 200 cc per breast is typically the upper limit of fat injection recommended by most plastic surgeons.  It is not that more cannot be injected; but on average, this has been determined to be the most that can be expected to live during one transfer.  Multiple transfers would be required for additional cc's.  In general, the abdomen and thighs are preferred donor sites but fat from other areas can be used.
Some of the retention rate will depend upon the amount of fat transferred, as smaller volumes will likely have a higher survival.  Occasionally, a patient can resorb a great deal of the fat despite optimal transfer procedures. I typically transfer from 250 cc to 400 cc or so per breast.  Thinner patients are outstanding candidates in my opinion, because the fat transferred is extremely resistant.  I place one incision under each breast and the fat can be injected to the locations specified by the patient.
Results are largely dependent upon the skill of the surgeon, his/her ability to remove fat without creating contour deformities, and your expectations. You would need a consultation with a board certified PS to determine your available fat stores.

At 5’6 and 129 lbs, your BMI is 21 and you are quite thin.  It may be hard to find donor sites however, I usually harvest from the anterior thigh.  You told us your cup size but not your bra size.  At size 32 or 34, each 100 cc of fat corresponds to 1 cup size.  At size 36 or 38, each 200 cc of fat corresponds to 1 cup size.  Fat transfer does not all take, but approximately 60% does.  You would need to have 150 cc of fat for a 100 cc gain for 1 cup size if you are 32 or 34.  However, removing your implants will most likely result in your breasts hanging.  You may benefit from a breast lift at the time of explantation using The Bellesoma Method.  The implants are removed, your breast tissue is reshaped creating upper pole fullness, elevated higher on the chest wall and more medial to increase your cleavage.  Vertical scars are avoided, nipple sensation (in 95%) and the ability to breast feed are maintained, if not injured during your previous surgery.  Best Wishes,Gary Horndeski, M.D.