Zone,you can retrain every unit, except your Daimyo,for upgrades of weapons,armor,honor,
etc.Here's how:
1.The unit to be trained must be in the province with the training dojo,armory,sword smith,etc. for one turn.
2.Left click on the "train troops" button to bring up the training parchment.
3.Left click on the unit you want to train on the strategy map.His icon will appear on the bottom of the screen.Click and drag the icon to the training parchment box.
4.Next season your retrained unit will be ready to deploy.

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Diplomacy is the art of telling someone to go to hell so that they look forward to making the trip.