What is Really the Difference Between a 450cc and 500cc?

The size of the implants are based on a combination of a patient's goals
and objectives and her anatomy. In order to provide a natural looking
augmentation, the surgeon needs to evaluate many factors including soft
tissue coverage, skin laxity, chest wall width, breast imprint width,
the shape of the breasts and relative level of constriction to ensure
that the breast implants are appropriate for your body. Consult with 3
experienced and expert board certified plastic surgeons to understand
your objectives.

If you are going under the muscle and getting a lift, I would recommend at least 500cc to meet the look of the 450 cc sizer.   In order to make an accurate size recommendation, I would need to assess your chest wall and breast mound measurements and characteristics.  Unfortunately, there is not a general rule of thumb or objective criteria to implant selection.
Your plastic surgeon will perform several measurements of your chest wall and breast anatomy and determine a range of implants that both fit your chest wall and reach your desired goals.
The next step is to try on this range of implants in the office with your doctor.   The key to this success is showing your surgeon the body proportion you desire with a bra sizer and allowing your surgeon to guide you to the right implant.   It will be much easier to communicate in implant cc's than cup size when determining the appropriate implant for you.
I wish you a safe recovery and fantastic result.
Dr. Gill