Breasts Sagging After Changing to Smaller Implant
You must have a really small frame as few 210cc implants are used. But because you are so slender a decrease of 20cc's may make a big difference. Yes the breasts will be more loose but they won't sag per se. The nipple will remain in the correct position but because the tissues are stretched out the breasts will appear less "perky." This can be corrected by a small periareolar mastopexy or breast lift.
Exchanging implants and drooping
First, a 20 cc drop in the volume will do almost nothing in terms of reducing the volume and certainly will not cause anymore sagging than what you have now. The question is if you think the size is too big why are you only going down 20cc's? Besides, the 210's are tiny implants to start.
Volume may not be the problem
I agree that 20 cc.s is relatively insignificant. If a patient is contemplating going smaller, then I usually suggest going down at least two sizes within the same model style to make it worthwhile and noticeable. However, your surgeon should analyze why you think the implants are too big. Is the the width of the breast? Is it the projection? Are they very close together? Do they cause fullness superiorly? Once you analyze your breast and why you feel they do not fit your idea of what would be attractive, then changes in the style, width, height and projection of the implant may be in order. These changes will dictate a volume. It is possible that you might need to go with a narrower implant, but higher profile, with little change in the absolute volume.
It won't make any difference.
The difference of 20 cc will not be noticeable. You are saying that you want to go down from 7 ounces to 6.3 ounces. That is hardly a change and I doubt that it will make any visible difference. By the same token, because the difference is negligible, your breasts are not likely to sag if you change to 190 cc. Be advised that replacing the implants carries many of the risks of the initial operation and there is a risk of infection and implant loss. So be sure you want to make that small of a change before proceeding.
Smaller implants and breast appeance
Going down in size by 20cc won't make much of difference at all, even for your breast size. Going down by 100cc will make a difference, and you may need a breast lift afterward if you areolae start to droop. Talk with your plastic surgeon about this and be sure to look at some before and after photos. Again, going down by 20cc may not be worth the expense and recovery!
Don't waste your time or money or health in downsizing by only 20 ccs - we are only talking about less then one ounce of liquid. If I gave you a 210cc and 190cc implant, you would be hard pressed to tell me which is the larger one (without cheating and looking at the label).
Enjoy the 210s.
Downsizing implants and risk of sagginess
I can only answer in general terms.
I believe that downsizing your implants by 20 cc is a relatively small amount and you should expereience minimal sagginess from that change.
That having been said, that is such a small change that you may barely be able to tell the difference which begs the question: Is it really worth it?