Breast implants are replaced for any number of reasons, sometimes for size change, more often later as the implant has worn and begun to leak. If the capsule is in good condition and has the right shape, replacement into the same pocket works very well, and the submuscular location of your implant is ideal.
There are several schools of thought concerning the pocket size in augmentation. Some believe that the pocket should be larger than the implant to keep the implant soft. Massages are designed to allow displacement of the implant within the pocket, usually upward. Others believe including myself that the implant should fit the breast well, be centered on the nipple and should hug the tissue and feel as one. The capsule should fit the implant.
It can be difficult to predict what the capsule will do in response to a smaller implant. It may fit just fine, it may shrink up a bit to fit, or it may not change at all. If the capsule is too large it can be reduced at the time. This need can be predicted by the displacement I find the most objectionable, a fall or sliding of the implant to the side when on your back. A low pocket is not very nice either and can be corrected as well as the size of the implant is reduced. Your surgeon should be able to anticipate and find the best fit. Don't let this discourage you from changing to a implant that is right for you.
Best of luck,