There is no question that you could change out your saline implants with silicone implants through the axillary incision. I do it every week.
You have to find someone who specializes in this approach, as most plastic surgeons do not feel comfortable with this incision, so they love to poo poo this approach. The facts are that I have done about 2,000 breast augmentations over the last 20 years, and have replaced 76% of the implants back through the armpit incision when needed.
The only times that this is not able to be done in my practice, is when you have to remove a portion of the capsule (scar) from around the implant (capsulectomy). You have to be able to see to disect between the capsule (scar) and the normal tissue, which is impossible through the armpit.
So for capsulectomies, you have to use another incision like the areolar or inframamary incision as much as it pains me. You just have to find someone who is expert at the transaxillary incision.
Don't give up. Good luck.