Size Down to 250cc Silicon? (photo)

Thanks for the question. Downsizing by 50 cc will not significantly change your appearance. If you downsized by 100 cc of more, then I suspect that you will need a lift but a physical exam would be required to confirm this. See a board certified plastic surgeon in consultation to explore your options.

Based upon the posted photos I would not decrease your size of implant. It would in my opinion give a "saggy" appearance. Seek in person opinions. 

If you are planning to decrease your implant size from 320 or 300 cc to 250 cc, and already have stretch marks on the breasts, it is possible you may need a breast lift. Whether or not the breasts look saggy after downsizing really depends on your skin quality, and how much sagging you already have. Based on your photos it looks like the nipple is in a reasonable position on the right breast, but maybe a little low on the left. Breast lifts can be done at the same time as changing out the implants, or you can wait and see how your skin responds. You can always do the lift later if needed. I recommend consulting with a board certified Plastic Surgeon in person to really see what is the best option for you.