Is it possible to upgrade my existing infantry, or must I disband the old units and build new ones?
I purchased the next military technology level which opened up Azab Infantry and made Yaya Infantry obsolete. It prompted me to choose the new unit type, but I cannot find a way to upgrade my existing infantry.
I'm in the middle of a Peasant War (endless uprisings) and have no manpower left. Will disbanding units put the remaining men back into the manpower pool?
When you change your default unit type (e.g. because of an upgrade to a better unit), all your troops will automatically "evolve" into that new unit type. There are no further actions you have to do about that. This will happen always when you change a unit type.
Keep in mind however that changing a unit type will give all troops that contain the affected existing units a severe drop in morale. This is due to the short-time chaos that comes with reorganizing a regiment.
For this reason, changing the unit type in the middle of a war can be a bad idea, depending on the situation of course.
