So, I already figured out that there is no upgrading a unit from since before you had an armory, meaning the ashigaru spearmen have to be disbanded and recreated from scratch. Also, the units you get as a reward do not have upgraded equipment even after you have built an armory(blacksmith for weapons or armor). Generals on the other hand, coming of age or otherwise, do get upgraded equipment, but the Shimazu katana samurai I got for taking Osumi, after I had the armory upgrade, did not receive better swords, though the General did, who arrived just before the turn I completed the mission, but still several turns after the armory was done constructing.

Do any mods work with this feature, so that levy troops such as spear and bow can upgrade and the units you get as mission rewards are upgraded? I'm ok with katana samurai not upgrading because it makes sense that they wouldn't simply discard their swords en masse for the piece of equipment they've dedicated their whole life to.