The American Academy of Pediatrics just released updated recommendations about the use of rear facing car seats. It is now suggested that parents put children in rear facing car seats until age two (Or even longer if the child is small) as opposed to the current recommendation that they ride in such seats until age one.
The reason for this change is that rear facing car seats distribute the force of an accident more evenly across a child's body, preventing head, neck and spine injuries in frontal crashes, which comprise the bulk of all accidents. The AAP policy statement sites research which has shown that children under two are 75% less likely to be severely injured in a crash if they're in rear facing car seats.
Most convertible car seats on the market can accommodate a two year old up to 35 pounds, so you don't necessarily need to buy a new car seat. The biggest fuss may come from your toddler, who may whine about having to turn around.