H. - here is an excellent resource for all things car/booster seat related. There is a wealth of information with regard to different seats as well as how to safely use them. http://car-seat.org/
That site, as well as personal research and discussions with car seat safety technicians, compelled me to keep my children rear-facing as long as allowable by the car seat & also to keep my front-facing children in a 5 point harness as long as possible. My nearly 6 year old child (53 pounds) is still in a 5 point restraint. If you do move to a booster, you'll want to consider a belt-positioning booster with side impact protection.
Also, regarding seats that pass from one child to another, remember, each seat has an expiration date (most seats are good for about 5 years). It should be marked somewhere on the seat, and, of course, never reuse a seat that's been in an accident.
Have a great day.