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.

If you want a smaller breast size, you should be able to have one.  It's hard to say exactly how many cc's would give you the optimal result.  Sizers are very helpful during the revisional operation, to determine the answer to that question.  The sizers may be placed, and you could be evaluated ( during surgery) in an upright position, prior to placement of the final implants.  If 250 or 275cc's give you an approximate B cup, either one could be used.  If the pocket is too large, or the bulk of the implant is too low, an excision and closure of parts of the capsule can be performed during the surgery.  This is called a capsulorrhaphy. With this technique, the capsule can be made smaller in the right places, so that you would have a  perky look, with nipples in an anatomically normal position.  Good luck.

The difference in the diameter between 325cc and 250cc implants is between 1cm and 1.5cm.

The apex (most projecting point) of the implant would drop approximately 1/2- to 3/4cm (~5-7mm).

It is unlikely you will see much change in the orientation of the nipple with that change: my best guess is it will be slight, if at all noticeable.