I am getting my implants swapped for a smaller size implant. Will my boobs sag? (Photo)

Your breasts are too low on the chest wall and changing implants will not be successful.  At this time, I recommend explantation and lift using The Bellesoma Method.  The implants are removed, your breast tissue is reshaped creating upper pole fullness, elevated higher on the chest wall and more medial to increase your cleavage.  Vertical scars are avoided, nipple sensation (in 95%) and the ability to breast feed are maintained, if not injured during your previous surgery.  Later, fat transfers can be performed if additional volume is desired.  Implants are not lifetime devices.  8% fail within the first year and 30% fail within six years.  Fat transfers are permanent and incorporated into the body.Best Wishes,Gary Horndeski, M.D.

Currently your nipples/soft tissue are lower than optimal, which means the pockets are holding the implants too high, as you said. Placing smaller implant would likely lead to better implant position and (possibly) improve nipple/soft tissue position. Why would the pockets need to be made even higher than? Your photos are unhelpful: elbows at the level of your shoulders, which lifts the breasts, and selfie mirror photo which distorts just about everything. If you really want an informed answer from an experienced revision breast expert, you’ll need to be seen. 

Overall I think you have a nice result. It should be simple to lower one implant a bit. I think your size is also nice. Switching to a different size implant will cost more and always comes with additional risks of asymmetry especially trying to close down the pocket. I would not recommend surgery that has so many variables and would simply drop the one side a bit if you want.