If your daughter is old enough for a booster seat you may want to check the date of manufacture on the infant seat you own. They expire after 6yrs.
Most convertible seats, though the weight says '5-xxlbs', dont' fit newborns well. while rear facing, the harness needs to come from just at or below the shoulders, and there are very few convertible car seats that allow even a large newborn to fit properly. Cosco Scenera and the Graco MyRide 65 are two that fit newborns well. First Years True Fit is another that fits newborns pretty well. Britax seats don't fit newborns well, the bottom harness slot is too high to be just at or below the shoulders.
Its wise to either find a convertible that will fit a newborn well (one I listed above) or go with an infant seat until your child is big enough to properly fit into a convertible.
Also, please read about Belle's gift and Kyle David Miller online. These are children who were in booster seats too early and died because of it. If your older child is younger than 4 AND 40lbs, she should still be in a harness.