How different would 390ccs vs 450ccs silicone round textured implants be in the body?

If you are going to have a breast lift, and want to be smaller, there is no better time to down size your breast  implants. The breast lift can be adjusted to match the new breast volume. Reducing your breast implants' volumes from 450 to 390 would be expected to reduce your bust about 1/2 a cup size. The lift alone will reposition the breast volume and reshape the breast, but not reduce volume. Decreasing the breast volume without a lift, may cause additional sagging. 

    Though a breast lift only removes skin and no breast tissue, the compression of the skin envelope on the breast does reduce the apparent volume.  This depends upon the density of breast tissue and the amount of skin laxity.  You will have to trust the judgment of the plastic surgeon you select. Select a plastic surgeon who performs hundreds of breast augmentations and lifts each year, has great reviews, and has great before and after pictures.Kenneth Hughes, MDLos Angeles, CA

HI Itsjustmeb.  The volume changes you mention are only about 5 to 10% of the volume of your current implant.  I do not think that this will make a dramatic change, since your breast mound is composed of the implant AND your natural breast tissue.  390 cc implants are still large, and I am sure you will still look pretty despite the reduction in size.  If you want to be a little smaller, then you will probably be happy. 

While there is a 60 cc difference in volume, it may not show up too much in the projection or width.  You can do a baggy test to see if you like the volume.

As others have stated going from a 450 cc breast implant to a 390 cc breast implant is not a very significant reduction in breast implant size.Since you're having a breast lift and want to reduce your breast implant size my guess is he would have to go down to 300-325 cc to see a significant difference.  However you need to have an in person examination and consultation with your plastic surgeon to carefully discuss this issue.

Going down in size from 450cc to 420cc would be a very mild change, perhaps barely noticeable.  If you feel that you are too large, as you state, you would need to decrease your implant size to at least 390cc to see a significant difference, though this is not a small implant. Unless your surgeon has told you that tissue will be removed, a breast lift alone removes only the outer layer (epidermis) of the excess skin, and thus should not result in any real reduction in breast volume. Therefore, if you are looking for a smaller breast size, you will need to reduce your implant size, and 30cc is a very subtle change.

In general, if you are starting out with 450cc and go to 420 you would see no difference. If you go down 100 cc you would be able to see a partial cup size change.