If you need your breast implants redone, it may be time to take a close look in the mirror and see if what you really need is a lift. If the implants are soft, they may already be large enough.
The second most important question now that you've lived with implants is ... do you really need them to be any bigger? A proper lift can be done irrespective of their size. Many think they need them bigger but are perplexed at why they are dropping so much.
Perhaps the descent of the breasts is related to the sheer weight of the implant. People's breast tissues drop even if they don't hold up an implant, so consider decreasing in size. The more you decrease in size the more your lift will be necessary.
Lastly, keep in mind that expecations may be the most important element in this particular operation. Spend some time with your plastic surgeon and review EXACTLY what to expect because this time it is very different than when you had them done for the very first time.