From Megami magazine:
"In this work there is a enemy that will face off against humanity in place of Fine. They are, astoundingly, the same as Hibiki's team. New "Valkyries" clad in the "Symphogear" armor, utilizing "songs" as weapons. However, the "Symphogear" is a highly confidential treasure that is held only by the Japanese Government. It should not be possible to create new Symphogears using the "Sakurai Theory," as Fine who was the founder of theory is now gone. But despite that, how have new "Valkyries" appeared? What is the goal of these girls who bear their fangs against humanity? The mysteries keep piling up."
Additionally here is an image of the announcement of Super Heroine Chronicle, you can see Hibiki's SD model in the bottom left. The SD models are used during the combat animation, then there will be small sprites on the world map (which you can see on the very far left) and then dialog scenes will have the regular anime designs (which you can see some from other series on the right).