There is no question that the transaxillary incision can be used to exchange implants. That has been my prefered method for the 20 years that I have been in practice. In those years, 74% of my revisions have been able to be done through the transaxillary approach the second time.
I agree with other physicians who have answered this question, as there are some other things to think about, such as how big you would like to go, and how big the pocket is currently. It is almost impossible to do a capsulectomy (removal of the entire capsule, which is scar around the implant) through the armpit, but you can do a capsulotomy through the armpit fairly easily (cutting through the scar to make the pocket bigger).
I have put in large implants through this incision (700 to 800), so I do disagree with saying that there is a "maximum" size that can be placed through the armpit. One of the biggest factors is how comfortable your surgeon is with that approach. The more familliar, the less trouble he or she will have using that incision for revisions.
Make sure that you find a Board Certified Plastic Surgeon to take care of you, and preferably one that is a member of the American Society for Aesthetic Plastic Surgery, as these surgeons do most of their work in Aesthetic surgery. Good luck with your search, and having a successful surgery and recovery.