Up to the 6th ed, heavy armour was virtually heavy hauberk or full plate, as there was no category for full plate (but Mithril and Chaos Armour). All lesser armour was just light armour, however heavy or minimal it was.
With the 6th ed, Empire got full plate for the new Greatswords, and this opened the can of worms about what the armour options for less armoured troop types were.
No, there never was an official way to upgrade State Troop armour. Light Armour is Empire standard. Even the 7th ed State Troops have visibly variable armour, one pose has no visible armour and another pose has rather heavy and ornate armour. And then the plastic Greatswords have armour that doesn't look like full plate, but rather more like heavy armour. So the look of the figurines and rules for armour do not go hand in hand.
But as long as you play with your friends, you can always agree on an armour upgrade. Heavy armour upgrade could cost +1pt for cheaper troops and +2pt for more expensive troops.
-Z