Acceptance, both self - acceptance and acceptance from others. They may face challenges from family, friends, and society at large. For example, in many stories, the newly transformed character has to struggle to get their loved ones to understand and accept them. And also there is often a theme of discovery. Discovering new aspects of themselves, new feelings, and new ways of interacting with the world around them. It's like opening a new chapter in their life where everything is different.
Identity is a major theme. The character has to figure out who they are now that their gender has changed.
Another theme could be the exploration of gender roles. When a guy turns into a girl, they get to experience first - hand the different expectations and limitations placed on women in society. This can lead to a deeper understanding of the social construct of gender and may even prompt them to challenge these norms.