There are several possible reasons to change breast implants.
It is normal for scar tissue to form around an implant. However, sometimes, this scar tissue is too much and it leads to hardness and possibly pain. The implants themselves NEVER get hard. It is the surrounding scar tissue that gets hard. We actually express it by saying, “The implants are hard” but it is actually technically not correct. If the scar tissue becomes “too hard” (which is clearly subjective) or the breast shape does not look as good as it did previously, it may be a good idea to release or remove the scar tissue or to create a new pocket for a new implant. New implants will usually be silicone if the previous implant was saline, though it does not have to.
The woman wants a size change – either to go larger (more common) or to go smaller. She may have wanted to go larger initially but the stretch would have been too much. Now that she has used a smaller size, the tissues have stretched. This usually means that a larger implant can be placed at this time. Or she may have just decided that she wants to go larger (usually if weight is put on then a larger breast implant will be needed to achieve the same effect). Or she may have decided that she wants to go smaller.
With time (and perhaps children and breastfeeding) there has been a change in breast shape (usually droopiness or ptosis) requiring surgical correction. If a number of years have passed since the original implants were placed, this may be an opportune time to change them out with this new surgery.
A significant amount of time has passed since the implants have been placed. I personally, do not feel that there is some magic date by which time, the implants need to be changed out. I have seen some of my own patients, in whom I have placed saline implants (under the muscle) over 20 years ago who still look great. If you look great and feel good, at this time, I think you can wait. Of course, you can do an MRI to look for any leaks with silicone implants. I believe the FDA is recommending you get an MRI, I believe every 3 years. I personally have mixed feelings on their suggestion.
Generally speaking, if you are just changing out implants that are the same size (or close to the same size), there should be almost no pain. This is because they are placed in the same pocket. Any discomfort will be related to the work done on the breast pocket, or new pocket formation or reshaping the breast. This should be quite tolerable. It is important to realize that this can be a very difficult surgery. Even with a lot of experience, a great result is not guaranteed. There is often no one “correct” way to deal with the situation and different surgeons may approach this situation very differently.