Redone completely.

In my headcanon, an alicorn's extra magic flows into their hair

Lunas rapid hair magic transformation was because of the imprisonment in the moon, her growth was halted, and magic built up, unlike Celestia, whose hair transformed after centuries, the magic released in hr much quicker, reaching full power in the twenty years or so since her release.

Celestia, on the other hand, had her magic grow to control the moon and had excess magic when her sister regained control, which changed her hair