When a child is rear facing the harness should be at or below the shoulders, this prevents the child from riding up the seat in a crash. When a child is forward facing, the harness should be at or above the shoulders. This holds the child against the seat in a crash. If the harness is below the shoulders in a forward facing child, the child is at huge risk for spinal compression, which is very dangerous, and deadly.
A forward facing car seat is outgrown when one of three things happens; the child goes over the forward facing weight limit, the tips of the ears are level with the seat back, or the shoulders go over the top harness slot.
Ther eare quite a few affordable seats on the market right now that will harness your daughter for quite some time.
Evenflo Maestro is $80 and will harness to 50lbs and has a taller top harness slot, my 5yr old still fits in the harness! It also becomes a really good booster seat after the harness is outgrown.
Evenflo Generations65 is $99 and will harness to 65lbs. It doesn't make a very good booster, but its a reasonable option that will harness your daughter until she's the appropriate age and weight for a simple booster seat.
Graco Nautilus is $150 and will harness to 65lbs, and will harness the longest because of height out of the three seats listed here. It also becomes a booster to 100lbs, and a no back booster as well. This is a GREAt seat, very comfy for kids ,easy to install and use.
Keep her in a harness if she is under age 5, but if her shoulders are over that top harness slot of her current seat, you must stop using it now and get her a new seat. the Evenflo Generations is at Babies R Us, the Maestro and Nautilus are at Target or Wal Mart.