When I had Madison we changed her car seat just before she hit 6 months she was 8 kgs then, but If i new what I know now I would of held it back a little, With Charlotte I have a S&S car seat that I can change her at 12 kgs, She is 8 months old today and is only 7 kgs, so I wont be changing her for a long time.
Also with booster seats there is a new one on the market called Secure, it hooks up to the car, as well as having the space to hook up a HR harness, Its a great car seat, but it carn't be used until 14 kgs.