If I replace 325cc for 250cc, will they sag down because a larger pocket has already been formed?

When exchanging larger implant for smaller implants there is some risk that the implant pocket would be larger than desired for the new implant.  A significantly smaller diameter implant would tend to look a little bottomed-out.  Just as important, when lying flat the implant can sit too wide on the chest.

That said, 325 cc to 250 cc is not a huge change.  If the original implant were anything but a moderate profile implant, I am sure that changing to a lower profile implant would make up for much of this difference.

For more significant size decreases, revision and tightening of the capsule may be necessary.

Downsizing your implants from 325cc to 250cc really should not cause much problem.  I use sizers in the operating room because it's difficult for you as the patient to know how many cc's it will take to achieve the results you want. Take in some "goal" photos and allow the surgeon to make the call as to how many cc's will achieve your goal.  Sometimes, when there is a big difference between the implants (going from larger to much smaller) some internal sutures may help with pocket placement.  See some photos of similar situations from your surgeon and make sure you discuss everything in detail.

The diameter of a 350 high profile silicone implant is 11.4cm and the projection is 4.9 cm. The diameter of a 275 cc moderate profile silicone implant is about 11.6 cm. and the projection is about 3.2 cm.  So, if you have a high profile implants and reduce the size to 250 with moderate profile, the pocket would be filled with the implant and the change of the shape should be minimal. If you have moderate profile 350 cc with a diameter of about 13cm and change it to 250cc moderate profile with a diameter of about 11.5 cm  you might have slight change in the shape of the breast bt I don't think it would be tha dramatic.