Maniac
Apolyton Sage
I'd like to make a list of free beneficial promotions you can get by upgrading to a unit from a lower tier instead of building the unit directly. Anything I missed? Especially with all the unique units it's easy to miss something.
General
Horseman -> Chariot & Horse Archer
Cover I
Royal Guard -> War Chariot & War Elephant
Defensive
Magic
Warrior -> Archer
Enchanted Blade
Religious units
Acolyte -> Confessor
Medic I
Crusader & Paramander -> Paladin
Medic I
Soldier of Kilmorph -> Paramander
Dwarven & Guerilla II
Thane of Kilmorph -> Paramander & Stonewarden
Medic I, Guerilla I
Disciple of Leaves -> Priest of Leaves & Ranger
Woodsman I & Medic I
Archer of Leaves -> Ranger & Longbowmen
Elven
The Drown -> Stygian Guard
Undead, Vulnerable to Fire, Water Walking
Zealot -> Stygian Guard, Cultist
Dwarf Slaying, Medic I
Savant -> Ritualist (& Mage)
Elf Slaying & Medic I
Unique units
Chanter -> Shadow & Marksman
Channeling I & Dimensional I
Dragon Slayer -> Berserker
Courage & Dragon Slaying
Hammerfist -> Dwarven Berserker
Dwarven & Guerilla II
Dwarven Slinger -> Crossbowman
Dwarven
Dwarven Soldier -> Paramander & Pikeman
Dwarven & Guerilla II
Firebow -> Flurry & Marksman
Channeling I/II, Fire I, Sorcery, Flaming Arrows
Guardsman -> Maceman
Guardsman
Harlequin -> Druid & Beastmaster
Channeling I/II, Sorcery, Mind I, Chaos I
Lightbringer -> tier2 disciple
Sentry I
Lizardman Assassin -> Shadow
Orcish & Amphibious
Shieldbearer -> Shield Wall & Spartiatoi
Guardsman
Taskmaster -> Marksman & Courtesan
Taskmaster
Vampire -> Brujah
Channeling I/II, Body I, Death I, Sorcery
General
Horseman -> Chariot & Horse Archer
Cover I
Royal Guard -> War Chariot & War Elephant
Defensive
Magic
Warrior -> Archer
Enchanted Blade
Religious units
Acolyte -> Confessor
Medic I
Crusader & Paramander -> Paladin
Medic I
Soldier of Kilmorph -> Paramander
Dwarven & Guerilla II
Thane of Kilmorph -> Paramander & Stonewarden
Medic I, Guerilla I
Disciple of Leaves -> Priest of Leaves & Ranger
Woodsman I & Medic I
Archer of Leaves -> Ranger & Longbowmen
Elven
The Drown -> Stygian Guard
Undead, Vulnerable to Fire, Water Walking
Zealot -> Stygian Guard, Cultist
Dwarf Slaying, Medic I
Savant -> Ritualist (& Mage)
Elf Slaying & Medic I
Unique units
Chanter -> Shadow & Marksman
Channeling I & Dimensional I
Dragon Slayer -> Berserker
Courage & Dragon Slaying
Hammerfist -> Dwarven Berserker
Dwarven & Guerilla II
Dwarven Slinger -> Crossbowman
Dwarven
Dwarven Soldier -> Paramander & Pikeman
Dwarven & Guerilla II
Firebow -> Flurry & Marksman
Channeling I/II, Fire I, Sorcery, Flaming Arrows
Guardsman -> Maceman
Guardsman
Harlequin -> Druid & Beastmaster
Channeling I/II, Sorcery, Mind I, Chaos I
Lightbringer -> tier2 disciple
Sentry I
Lizardman Assassin -> Shadow
Orcish & Amphibious
Shieldbearer -> Shield Wall & Spartiatoi
Guardsman
Taskmaster -> Marksman & Courtesan
Taskmaster
Vampire -> Brujah
Channeling I/II, Body I, Death I, Sorcery