Exchanging 350cc for 250cc Breast Implants with Minor Breast Lift?

Make sure you know exactly what the surgery entails.  It is fairly easy to determine how much native breast tissue you have, and how your likely postoperative appearance would be with a lift and smaller implant.  However the difference between the two sizes you mention is not huge, so do spend some time with your surgeon on this issue.
You don't want to have a surgery only to discover you are still top heavy and need further surgery.

Hard to say without seeing you.  Having said that, often when we down size implants there is extra skin.  This skin will sometimes need to be addressed with a lift.  Talk with your Plastic Surgeon about your expectations.  Bring in photos if you can.  I also would wait at least a year before doing revisional surgery.

Hi there-
The best advice I could give you is to focus on your desired breast contour.
Forget everything else.
Close your eyes and imagine the breast size, shape, and proportion to the rest of your body that you would like to achieve.
Now communicate that appearance to your surgeon. In my opinion, the best way to do this is through photos (that whole "picture is worth a thousand words" thing has been around a long time for a reason), but the point is really to communicate what you want to LOOK LIKE- NOT what number you want on your implants or what letter you want on your bra.
After understanding your desired APPEARANCE, your surgeon should be able to make a good recommendation for achieving it for you.
For more about how to properly proceed, and how not to, please read this:
http://orlandoplasticsurgery.blogspot.com/2010/01/achieving-beautiful-breasts-you-desire.html#f562 

The one question I would ask is how long you have had the implants? If they have been present for over a year the breast tissue has accomodated to the new volume and your skin will not tighten after the implants are removed. 350 cc are large and the impact on the breast and the skin will be obvious upon removal.
Generally speaking, when implants are removed the upper pole volume of the breast is dramatically reduced. The nipple position will appear lower and the breast will appear "saggy".
Like the surgeon you saw in consultation, I would encourage decreasing the volume of the implant not removing it. The exception would be if you have ample breast volume, you are young, it is within 3 months of implant placement or the skin quality is excellent and you are accepting of a more "mature appearance" of your breasts.
If you have saline implants and are committed to a surgical procedure, your surgeon may be willing to "pop" your implants. This will give you the closest approximation to your appearance if the implants were to be removed.
I hope this was helpful.

The real question here is: Are you that dissatisfied with your size that you prepared to go back to your original appearance? If yes, remove the implants and consider a lift. If not, a 250cc implant is a decent size decrease and will not differ tremendously from a 225 or 200cc implant. If you are considering smaller than 200cc you might as well remove them completely.