As soon as DD got too heavy to lug around in the infant seat (4.5 mo), we got a convertible seat and leave it in one car and the infant seat stays in the other. It's pretty convenient!
You can switch whenever you want. Just make sure that whatever seat you get is truly sized for your LO and the harness straps will be at or below their shoulders (some seats are better than others for this). Many people never use a bucket at all!
Re: Car seats!