Will 250cc Breast Implants Be Significantly Different from 350cc?

Breasts that are lifted often look smaller. Combining a breast lift with a small implant such as 250 should achieve a full C-cup in the average patient. However, your alternative is to do a lift without an implant depending on the amount of ptosis present as well as the size of the existing breast tissue..

You will likely notice a significant difference between the 350 cc implants and 250 cc implants. It is possible to remove the implants altogether, but keep in mind that having those implants has stretched your skin, and so does pregnancy. Removing 350 cc implants altogether will result in a dramatically different breast shape and size, making you not just smaller, but possibly flatter and more droopy. This is especially true if you do not have much breast tissue and already have drooping. There are other factors involved too, such as the size of your frame, style of implant, quality of skin, etc. Having 250 cc implants will definitely look different from having no implants.
It is difficult to say without consulting with you in person what would be best, but what your surgeon has recommended sounds reasonable. Regardless of how small you decide to go, you want to have an attractive breast shape overall. Basically the options are implant removal +/- breast lift, and implant exhange +/- breast lift. Make sure you understand the expected outcomes and limitations for each option. Once your expecations are clear to your surgeon, he or she can explain how to best accomplish your goals. If you are uncomfortable with one surgeon's recommendation, consider seeing another board certified plastic surgeon for a second opinion.
Good luck!