How to downgrade armor?
What happens when a Samurai with armor does not wear all of it? Can he or she, as a conscious decision, decide to not wear all of their light armor making it, in effect, ashigaru armor(Giving +3 to TN2BH but not affecting athletics/ stealth rolls)? Could they "downgrade" their Heavy Armor to Light or Ashigaru?

They was rules about partial armor in first edition. You can easily downgrade an heavy armor to light armor, since it's just about removing pieces.