I would also wait until she outgrows it. You paid good money for that seat and it keeps her safer than a booster, especially since she is so tiny - why would you get her a new seat just to feel like a "big kid"? That really should not be a primary concern for choosing a car seat.
My 5 year old switched to a high back booster in DH's car when she hit 40 pounds, the limit of the convertible we had in that car. In my car she is still in her convertible that goes up to 65 pounds. I can tell that we probably only have 6 months left in it as she is reaching the height limit... But I will keep her in that seat as long as I can.