Implant exchange from 450cc's to 650cc's

In my opinion, your desire for a fuller look likely can be met without going larger, but going to a plastic surgeon that specializes in breast surgery.  Going larger as you have stated will result in complications most likely as you age and gravity takes its toll. 

450
cc implants weigh approximately 1 lb each and 650 cc implants weigh about ½ lb
more. The additional weight will cause skin
stretching resulting in inferior and lateral displacement requiring
revision. The other alternative is
reshaping your existing breast tissue with a new technique called The Mini
Ultimate Breast LiftTM. Using
only a circumareola incision it is possible to create upper pole fullness,
elevate them higher on the chest wall and more medial to increase your
cleavage. Aligning the areola, breast
tissue and implant over the bony prominence of the chest wall maximizes
anterior projection without changing out the implants. This may give you the look you desire while
avoiding the ugly vertical scars of the traditional technique, maintaining
nipple sensation and the ability to breast feed. Best Wishes,Gary Horndeski, M.D.

Without an exam it is impossible to determine if larger implants will cause greater problems for your particular case, but in general, larger implants do cause more problems.  Larger implants are heavy and this will result in more droopiness and stretching of the skin which will make the implants more visible.  But you're right, you will have to weigh those risks with you're desired look.  Make sure to discuss your concerns with a board-certified plastic surgeon.