Sounds perfectly normal to me. Lots of children, especially girls, decide to make changes to their names.
Sometime they swap middle for main name - like our Prime Minister -. His name is Alexander Boris (and a few more names) Johnson. At sometime he decided he wanted to be called Boris rather than Alexander.
My DD, didn't change her name, but did change its spelling - Jane to J.ne - (no that is not her name).
Your grand daughter probably doesn't like her first given name, but didn't know she could change it until she heard her aunt was doing it and then, with quiet relief, decided to shed her first name and use her second name.
How everyday. How normal.