I believe they did say it is one team for the whole Elite 4. However, there is technically no Champion and the battle with Lin is speculated to be within the Citae Arc-d'Astrae and not at the Pokemon League, so you'll likely be able to change teams before her. There has also been a new consumable item announced that acts as a portable PC, but it's not clear whether its availability is exclusive to the postgame.
Better start EV training those Pokemon :D