To change for a man is risky because it would mean to become a different woman from what you really are, for a short period before returning to your true self and also, how would you feel in the shoes of the woman of class? Perhaps not too well, since you describe yourself as a sporty girl, but I suggest that you find a compromise: your boyfriend should learn to appreciate you as you are and as you were when he first met her, but you may, in some cases, wear stylish clothes to make him happy.