Breast implant revision surgery

Has anyone had breast implants and got them replaced with bigger etc?
I am seeking advice from people in Melbourne preferably, with trusted surgeons.
I have had my current implants for 13 years and after breastfeeding 3 kids they have lost shape.
I want to go up a size but also replace due to the age of them.
what costs would I be looking at?
And did you go under the armpit to avoid scarring?
I get keloid scars so my old original under breast scars are still very noticeable.
Thanks!