There is no global storage, either. There is no storage at all in Elemental. To train a unit with plate armor, you'll need the resources, appropriate facilities (presumably) and time. No stockpiling, nothing.
If the econ planned has no Global storage for natural & manufactured goods, it means Plate Mail will not be manufactured until gamer request an upgrade. X Plate Mail will be produced per Smithy, and gamer should be able to build several smithies adjusting the LOCAL production rate.
I put more thought to it in the "Design based equipment upgrade" mechanism. This version offers more flexibility.
1. When production of Plate Mail become possible, an "Equipment Update" screen pops up.
2. In this screen, it will shows all the Unit designs that are capable to wear the Plate Mail. In OP's case, it will look like this:
Design\Swordsman I: Leather
Choice A} Upgrade Leather to Plate in this design
Choice B} Add new unit design Swordsmen II, who wears Plate mail instead
Design\Calvary: Scale mail
Choice A} Upgrade Scale to Plate in this design
Choice B} Add new unit design Calvary II, who wears Plate mail instead
4. If he choose A}, ALL existing Swordsmen I & Calvary stacks will have the Plate mail upgraded automaticaly
5. If he choose B}, a "Troop Management" screen shows up. It shows a list of all stacks you control. Gamer assigns exactly which stack will (or will not) be upgraded to Swordsmen II or Calvary I.
What about upgrade caravans? You choose to upgrade your units, and a city produces the equipment and a caravan is sent. When it reaches them, they are upgraded.
Yes, I had the same idea a while back. This feature will be awesome. On top of relieving gamers from cycling to all stacks requiring the upgrade, it offers a chance for opponents stealing your Plate mail. It does offer another dimension to the game. Finally, stacks further away from your smithy will receive the goods later.
I do think this new version of "Design based equipment upgrade" mechanism will reduce micro, even we'll be have lots of new equipment upgrade, unit designs or high number of units. Will you like it? Or what is the weakness?
(Finally, I definitely dislike the idea that when I can start producing Plate Mail, ALL of my units will be automatically upgraded with it. It will be way simpler to implement, but hopefully, Stardock will not use this shortcut.}