Symphony of War is extremely flexible for classes: no class change is permanent except for the dragon tree. There is also a proof of merit item that gives bonus stats in exchange for locking a character in their current class, but you can choose when to apply them. Resources are released when you change classes, so you can experiment freely without wasting anything.
Note that resources in the table below are given in the total required for a particular class, not the amount needed to promote. The promotion cost will differ based on which class they are coming from. Technologies are required for some promotions; these are bold in the tables below.
Male Classes
Class Tier Promote From Requirements Fighter 1 Bowman 1 Soldier 2 Fighter STR 25, Iron Spearman 2 Fighter STR 21, Iron Cavalier 2 Fighter STR 21, Iron, Horse Horsebow 2 Bowman STR 20, Horse Scout 2 Fighter, Bowman SKL 22, Horse Skirmisher 2 Fighter, Bowman SKL 20 Rogue 2 Fighter, Bowman SKL 25 Crossbowman 2 Fighter, Bowman Crossbow Forging , STR 18, Iron 1Apprentice 2 Fighter, Bowman MAG 10, Gem Champion 3 Soldier STR 47, Iron 2, Obsidian 2 Sentinel 3 Soldier, Spearman STR 42, Iron 3 Knight 3 Soldier, Cavalier STR 45, Iron 2, Obsidian 1, Horse Zweihander 3 Soldier, Cavalier, Skirmisher STR 42, Iron 2 Centurion 3 Spearman STR 45, Iron 2, Obsidian Hussar 3 Spearman, Cavalier, Horsebow, Scout SKL 42, Iron 2, Horse Ranger 3 Scout, Rogue, Skirmisher, Crossbow SKL 38 Assassin 3 Rogue SKL 45, Obsidian 2 Fire Mage 3 Apprentice MAG 45, Gem 2, Sunstone Lightning Mage 3 Apprentice MAG 45, Gem 2, Sunstone Ice Mage 3 Apprentice MAG 42, Gem 2, Sunstone
Female Classes
Class Tier Promote From Requirements Medic 1 Militia 1 Siren 2 Medic MAG 20, Gem Swordfighter 2 Militia SKL 20 Hospitaller 2 Medic, Militia MAG 20, Horse, Gem Acolyte 2 Medic, Militia STR 18, Iron Priestess 2 Medic, Militia MAG 20, Gem Sorceress 3 Siren MAG 50, Gem 2 Swordmaster 3 Swordfighter SKL 60, Iron, Obsidian 2 Paladin 3 Hospitaller, Acolyte, Priestess STR 35, Iron 2, Sunstone Valkyrie 3 Hospitaller, Acolyte, Priestess STR 25, Iron 2, Sunstone, Horse Templar 3 Acolyte, Priestess MAG 56, Gem, Sunstone
Unisex Classes
Class Tier Promote From Requirements Archer 2 Fighter, Bowman, Militia STR 18 Warbow 3 Archer, Crossbow STR 38, Iron 2, Obsidian 2 Raider 3 Archer, Horsebow, Crossbow STR 38, Iron, Horse, Obsidian Samurai 3 Archer, Horsebow, Crossbow STR 45, Iron 2 Gunner 3 Any Tier 2 class Gunsmithing , Iron 2, PyrociteSiege Cannon 3 Any Tier 2 class Gunsmithing , Iron 3, Pyrocite, HorseDragoon 3 Cavalier, Scout, Hospitaller, Horsebow Gunsmithing , Iron 2, Pyrocite, Horse
Dragon Classes
Class Tier Promote From Requirements Drakeling 1 Dragon 2 Drakeling Red Dragon 3 Dragon MAG 42 Silver Dragon 3 Dragon STR 62 Blue Dragon 3 Dragon SKL 40 Red Dragon Rider 4 Red Dragon Dragon Bond , Gem 2Silver Dragon Rider 4 Silver Dragon Dragon Bond , Gem 2Blue Dragon Rider 4 Blue Dragon Dragon Bond , Gem 2