If your breast implants are soft and freely mobile, I'm not sure implant replacement will improve your pain. The most common cause of breast pain with old implants (saline or silicone) is capsular contracture, which is an overgrowth of scar tissue around the implant. With capsular contracture, the implant typically feels too firm, may look too perky, and can be associated with pain. If this is the case, removing your current implant including the scar tissue around it and replacing it with a new implant may help. Go see your plastic surgeon for an exam and discuss the issues you're having.
It is not possible to diagnose your problem without a thorough examination in person. There are many possibilities for your pain, one of which could be related to your breast implants. Your implants are 18 years old and saline, it is time to either get new implants (upgrade to silicone) or just have them removed. Please consult with a board certified plastic surgeon who specializes in breast implant surgery to assure the best possible results.