I would keep her in her infant seat for as long as possible, up to the weight limit, or until the top of her head goes above the seat - for safety reasons. I leave my infant seat in the car - my LO is 11months old & still fits in it. The rear facing mode provides better protection for their neck muscles, in the event of a crash.
I think you can get 0-4 birth seats, have a look on the Britax website. The seat is in rearward facing mode for infants, then you change it round to forward facing for older babies/toddlers.