Dear H. S. …
I’m a big believer in consistency, which I think helps readers focus on what really matters in a story without distractions. Random name-switching is soaked in distraction potential. Once you introduce the nickname into the narration, stay with it. Now, if there’s a strong theme-, style-, or story-related justification for switching, okay, fine, do it—just make sure the switching contributes to the reading experience more than flusters it. Even devices intentionally deployed can hurt instead of enhance. If you want some elbow room on this, you’ve got it with the other characters, who can switch between the given name and nickname according to their personalities and relationship to your character, just as friends and family will do in real life. It all boils down to facilitating reader focus and enjoyment.