Will Breasts Sag when Replaced with Smaller Implants?

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!

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?

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.

 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.

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.