The E3 2013 trailer still showed scenes that made it seem like the invasion of Insomnia was supposed to be part of the game & mentioned the whole Versus Epic multi-game thing, so obviously the changes to characters/script caused by trying to cram some kind of version of the thing into one game and thus probably the decision to make the invasion of Insomnia into the Kingsglaive movie were made post-E3 2013.
I imagine maybe they 1) came to the conclusion that they wouldn't have time to make the OG intro in PS4 level quality while also developing the rest of the game in a timely manner (since the intro seemed like something that might be filled with setup, setpieces & all kinds of heavily cinematic shit, so very time-consuming to make), and 2) also didn't want to disappoint fans with completely disregarding the exciting, action-packed invasion that the very first VsXIII trailers already teased, so they came to a decision to rip it out of the game and make it into Kingsglaive movie with a partly altered story to fit the new narrative (taking Noctis out of the city and inserting a few new protagonists we can follow through the chaos).
I was starting to get a bit worried about what had happened to the OG intro, since the timeline of TGS 2014 & beyond trailers didn't quite add up to what we knew of its timing, though they still showed Insomnia and its magic barrier so obviously it wasn't completely removed from the story.