Is it true that fat can migration down and cause sagging after facial fat transfer?

1) fat will migrate down just like how all of your tissues migrate down as you age. Thanks gravity! But fat will be no different than any other tissue on your face, like your natural facial fat, your skin, your muscles, etc.
2) It would be highly unusual to see significant fat growth if, for example, you were to suddenly gain a lot of weight after the surgery. It is possible to get fat loss if you undergo major weight loss because that fat will be broken down the same way fat anywhere in the body would be. Usually this is more of an issue if you are doing it very close to the time of surgery.
3) You can get fat from anywhere in the body.

No drift with fat grafting. It is permanent movement of fat cells, so if you gain or lose weight in the future the area will get bigger or smaller. I do not like fat graft to calves, implants are a more reliable and powerful solution.