Reddit mentions of When Aidan Became a Brother

It's a little young for YA readers, but there is When Aiden Became a Brother

Sometimes people look like boys when they're little, but then they grow up and it turns out they're really girls on the inside, or they look like boys when they're little but later it turns out they're really boys.

There are also some books for young children that explain what it means to be trans in an age-appropriate way, which may be helpful. E.g., When Aiden Became a Brother, and It Feels Good to Be Yourself.