I might not be much help but there was something on the news tonight about under 4yo's need to have the harness still and 4-7yo's need to be in a booster....I didn't really hear much of what they were saying, maybe google it?
Jayden was 2yrs2mths when Ashlea was born, we wanted to upgrade him and give Ashlea his seat but decided in the end to just but a new one for Ashlea. Jayden is 14kgs as well and the seat is good until he is 18kgs, I'd say it will last him at least until for or beyond.