Leather armor and breastplates are different items (with different body coverage) and they cannot be worn at the same time, so short answer is you cannot get them to reliably drop a leather armor when a metal breastplate becomes available, even though a uniform item like "gauntlets" will upgrade from bone to copper to steel when the item becomes available.
But, you are equipping 160 guys and are likely working with squads of 10 all the time, so try this...
Make two uniforms that are almost duplicates, one with leather armor and one with metal breastplate. Everything else in the uniform would be whatever-material, so the gauntlets would be "gauntlets", not "metal gauntlets". Then craft metal breastplates in groups of 10 using the job manager. When you see an order for 10 metal breast plates is done, go to a squad that has the leather armor uniform and assign the entire squad the metal breastplate uniform. They will replace the leather with the metal and keep the rest of their stuff (because the rest of their stuff is already the "most valuable" of whatever-material).
If you do this, I'd suggest having a minecart QS stockpile for all armor and weapons, because some military dwarf will haul a corpse to the refuse stockpile, and as the first step of his next job of upgrading his exceptional steel helm to a masterwork steel helm, he will drop that exceptional steel helm on the refuse pile and leave you cursing when you find it has become a Xexceptional steel helmX after sitting on the refuse stockpile for a while.