Our state's G/T program stipulates that kids are to be retested every three years unless the parents and the school both agree that it's not necessary. So in a case like this where the school insists it's necessary, my DD would be retested.
I wouldn't worry too much in your shoes. Your daughter has shown no signs of regression. As you said, these things don't vary all that much.
What I would do, however, is point out the outrageous hypocrisy in your principal's stance, because if every kid has to be tested from K-8, and your kid was tested in K-8, they're treating her differently.
