Okay....when do you know to upgrade your child from carseat to booster?? DD is 3 and has a Britax marathon that goes to 65 pounds - she just tips the scale at 31 pounds. But she is starting to "look" too big for it. I plan on getting the Britax Parkway when I upgrade - but when should I move her up? Should I go ahead and wait until she is officially too tall for the Marathon? By law she will be in a seat for at least another 3 years still.