It's interesting how a central theme in this "middle school arc" of Goldfish Kingdom seems to be the struggle to leave/escape a past identity. Mikaze attempts to escape her old self by becoming "coldhearted" towards Yuka and joining the track and field club as part of her desire to become an ideal human. Ms. Iwai used to be passionate about supervising the track and field, but is now much more enthusiastic about the literature club.
The character profiles on the Mozocry website's Goldfish Kingdom page also repeat this theme. According to these profiles, the track and field club vice president starts competing in shot put to avoid having to compete in sprints with the club president, much to president's disappointment. The profile for Mikaze's new friend in track and field states that everyone assumes that she loves bears, and she's gifted bear-themed accessories. However, she's actually indifferent towards bears and her favorite animals are cats.
I'm guessing that this arc (assuming Imitation Crystal ever continues it) will explore different questions related to this theme. Does remaining your "true self" mean that you stagnate and can't move on from the past? If you push yourself to change, how do you tell if you're changing for the better or for the worse? How do you reconcile with the fact that you might drift away from people who you were close with? Whatever happens, I hope Mikaze and Yuka are still able to remain friends...