Wow, wow... long time since I last came to this forum and now it's freaking intense here. I can't stop laughing at the "Nobody Teory".
I think there will be a great mystery around Stella and Luna or maybe It's just a name change to make more obvious the relation with Noctis name.
Not everyone knows that "Stella" means Star but I think It's pretty obvious that Luna means Moon.
If it's true, I think they changed the background of Noct and "Stella/Luna" too and that scene of meeting in the top of the building with the Etro painting is now gone.
Like someone said before, some things probably changed from Versus XIII to XV and I think now they know each since they're kids. The fact that the "kid Stella/Luna" uses the exact same phrase that the "old Versus XIII Stella" uses maybes indicate that they changed the scene.
Now all that discussion remember me one old picture of "Stella" with a large moon and different clothes... Maybe this thing of Stella and Luna exists since the Versus XIII and no one noted. Can they really be two different characters?
Sorry If I wrote something wrong, my english is pretty rusted.