J.M.
I would NOT buy a booster seat for her yet. Although she is older and does meet the min weight for some boosters, you always want to keep them buckled in a 5-pt harness as long as possible. Most convertible car seats harness up to 40lbs so she still has a while even in them. And then they will be a booster up to 100lbs, so you will get plenty of use out of it. Its up to you if you have the money to buy another set of Britax seats but you can also find quality convertibles for less. My son is in a Evenflo triumph advanced and he loves it. It harnesses up to 50lbs or 5 years. So your daughter will have plenty of use out of it, and it only is about $130 at walmart.