5 years since breast lift - can I safely go from 550cc to 1050cc implants?

The risk of complications from breast augmentation is directly related to the size of the implants. Larger implants mean greater risks. 1050cc implants are very large and heavy. Be sure to have a thorough discussion of potential problems with your surgeon before you decide to go this large. Among anticipated problems, the most likely is recurrent sagging from gravity acting on the weight of heavy implants.

Dear ioxy,
you already have large implants and 1050cc are huge. Most surgeons wouldn't recommend you this due to very high risk of complications related to this size implants.
Daniel Barrett, MDCertified, American Board of Plastic SurgeryMember, American Society of Plastic SurgeryMember, American Society of Aesthetic Plastic Surgery

Nothing is safe about 1050 cc implants! Please do not even consider going that large. Since you have had a lift and are doing well, just keep what you have. Sagging tissue, and you were sagging initially, does not support heavy implants well.