Carseat

I have 2 kids right now and expecting our 3rd. I have an infant seat for the beginning but I'm thinking about instead of buying a convertible car seat when the baby gets bigger just using my oldest's convertible car seat and buying him a booster with harness since they're cheaper. He will be 5 when a month after this baby is born and probably close to 6 by the time this one needs to switch seats. Do you think that makes the most sense?