In the ATR from last year when Tabata was talking about the changes done from Versus to XV he mentioned that the first thing they decided and nailed down when rebooting Versus into XV was that XV would be a single game and would condense Versus and its sequels into one game, and that decision is directly why they made the changes from Stella to Luna. That would mean that the changes from Stella to Luna and the story changes happened in 2012 when Versus rebooted into XV, and both the 2013 trailer and Nomura's interview in 2013 support that.
First of all back when it was Versus it was stated by Nomura that Noctis only meets Stella for the first time as adults during the peace treaty party, that already means that the young girl we see with Noctis in the 2013 trailer cannot possibly be Stella and that Stella's role was already changed into Luna's, and the young girl we saw in the 2013 trailer was already Luna, and that is also again supported by it being confirmed that Noctis and Luna have known each other since childhood and with Luna in the Dawn trailer being identical to the young blonde girl in the 2013 trailer, same dress and all. But for Stella it was specifically stated that they meet for the first time as adults and we even see them introduce themselves. This means that Nomura was already changing Stella into Luna in 2012 and she was already changed by 2013, the star earrings are simply a remnant of an intent and doesn't really confirm she was still Stella, Luna wears a necklace with a star and moon on it with the same design as the star earrings.
Then comes Nomura's interview in 2013, in the interview the only thing he said was that back in 2007 when he created Stella that she wasn't his type of character and so he changed her into someone he now likes, mind you he didn't say the girl in the 2013 trailer was still Stella, he only said that in 2007 he didn't like Stella when he created her and so he changed her into someone who he now likes, people only assumed that the blonde girl in the 2013 trailer was still Stella because we couldn't have known any other name at the time.
This basically confirms that Nomura was directly responsible for Stella being changed into and replaced with Luna, then there is the obvious design change where the 2013 blonde girl looks practically identical to Luna, in fact there is less difference in design between them than there is 2013 Prompto and 2014 Prompto, and also the fact that Nomura is main character designer and Luna is a main character, so obviously he would have been the one who designed her anyway.
This leads onto the next point about the sequels for XV, people have completely misconstrued what Nomura said in 2013 and meant for the "World of the Versus Epic" to be, he said that they have no plans for a sequel, he said that XV will have it's climax but only that they could tell other stories in the games world, he didn't say they would be sequels, and that really isn't any different to what Tabata has been saying, like they could do something about the guys in highschool as a sidestory separate from the game itself.
Also Regis's role wasn't changed, the only thing about him that was changed was his face and hair design which isn't any different than Nomura changed Prompto in 2013 to 2014, the magic barrier "the anti-armament runewall" around Lucis had become known as a plot point again since 2013 in the official story description. Even in the JF15 trailer we can see the magic barrier around Lucis and in that same trailer they are still using the old CG render for Regis, really the only thing we can take from this is that they simply had not changed his face or hair in the CG scenes yet and so they didn't say anything at the time, he's still wearing the same Roen suit too.
Finally in the 2013 trailer we see footage of Noctis fighting in Lucis, but again that doesn't contradict anything we know as it is now because Tabata had already confirmed that there will still be gameplay in Lucis and it's just at a different point in the game than it was when it was Versus. The treaty signing was not cut, the only that that was changed was that Noctis is no longer in Lucis when the treaty signing is happening, the treaty signing still happens and it's during that when Lucis gets invaded, Noctis is on his way to Altissia when Lucis is invaded. The Lucis footage we saw in 2013 doesn't necessarily have to have been during the invasion, since Niflheim would have occupied Lucis anyway so anytime even if end game were Noctis to go into Lucis he would still be fighting soldiers and such in the streets.
As for Tabata saying XV isn't Versus XIII, well that is something we've known since the 2013 trailer when Versus XIII shattered and became Final Fantasy XV, Versus XIII became Final Fantasy XV, it's not a spinoff anymore it is now a mainline title, and Tabata has already gone on record saying that the core of Versus is still very much alive in XV and that XV is a much bigger game than Versus was, even in that same ATR he mentioned that they are still sticking as close as possible to the Versus story and that the only changes that were made were the one to Stella to Luna, and that resulted in them altering the intro in Insomnia so Noctis isn't in the kingdom when it's invaded, that change just means that instead of Noctis being in Insomnia when it's invaded he is instead on his way to Altissia for his wedding with Luna, the wedding itself which is part of the peace treaty, and since Noctis already knows Luna since childhood there is no point in Noctis being at the party in Insomnia just to meet her, which seems like the main reason why they altered it in exchange for Noctis going to Altissia instead, however Luna is still in Insomnia during the invasion and it's after the invasion that she is heads to Altissia and is waiting to meet up with Noctis, this really doesn't contradict a single thing we see in the 2013 XV trailer or any XV trailer since.
It's already apparent that the majority of the changes when Nomura was still around directing things, including the design changes, it just wasn't until after he left that they revealed the changes.