To change a character’s class, he must be level 10+ in his current class and have the minimum stats for his new class. Additionally, allowed class changes are limited to three outgoing choices, according to his current class. (Specific allowed changes below.) Recruitable NPCs cannot multiclass.
Stats are reset to the minimum on class change. You keep your skills. You also keep your spells, as well as your unspent spell-scribing credits. I’m not sure what happens with level progression; it’s not reset to 0, but it also doesn’t appear to be kept the same as your old profession. It looks like the progression table for the new class is used, with some multiplier. (Around 1.25x the natural XP progression cost, as far as I can tell.)
Bonus attribute points are retained, so if you are motivated and insane (hello, Codex), you could conceivably grind up to the minimum stats for the new class and then hoard your points until the switch, then dump them all. Could make for a rough start, though.
Outgoing class changes allowed:
WARRIOR: Berserker, Pirate, Ranger
WIZARD: Necromancer, Sage, Thaumaturge
CLERIC: Bard, Sage, Templar
THAUMATURGE: Necromancer, Sage, Wizard
NECROMANCER: Sage, Wizard, Thaumaturge
RANGER: Warrior, Thief, Assassin
THIEF: Bard, Pirate, Assassin
TEMPLAR: Warrior, Cleric, Bard
BERSERKER: Warrior, Pirate, Ranger
JESTER: (why???) Thief, Bard, Assassin
BARD: Cleric, Jester, Sage
METALSMITH: Warrior, Berserker, Pirate
PIRATE: Thief, Berseker, Assassin
ASSASSIN: (why?) Warrior, Thief, Berserker
SAGE: Cleric, Wizard, Thaumaturge
A pirate requires the following minimum stats:
JESTER! wow this guy has high requirements
Templars: If you want a templar, you have to either roll one as a durendil or multiclass in from cleric.
Assassins: Two of the four classes that can change to assassin are elite ones (Pirate and Assassin), so if you want one in your first 20 levels, you need a ranger or thief.
Pirates: This is the most accessible non-creatable class, with routes available from Warrior, Berserker, Thief, and Metalsmith.
Jesters: Only bards can become jesters.
Metalsmith: No one can change into this class; it can only be a starting class. Since it and Sage are the only ones who get metallurgy, and Sages get very few skill points to invest in it, you probably need to create a metalsmith if you want metallurgy.
Pure mages are mostly locked into staying pure mages; only clerics have exits. (Bards and templars.)
Martial classes generally have hybrid advanced classes available.
All classes in Grimoire learn spells. There are 5 basic spell schools associated with a specific specialist who starts learning that school at level 1, as well as Arcanum/Eldritch spells. Each class begins learning from a specific list at a specific level.
Spell interactions with multiclassing: Spells available to learn on level-up are based ONLY on your current class. The list of spells available does not transfer between classes. E.g., if you are a level 10 cleric and multiclass into templar, you do not get spell picks for your first two templar levels, and when you start learning spells from the Cleric list at templar level 3, you only get the spells available that a level 3 templar (level 1 Cleric) would get.
Base caster classes:
WIZARD, CLERIC, NECROMANCER, SAGE, THAUMATURGE: Each learns his own school, starting at level 1.
WARRIOR: Wizard spells at level 9
RANGER: Thaumaturge spells at 4
THIEF: Thaumaturge spells at 6
TEMPLAR: Cleric spells at 3
BERSERKER: Thaumaturge spells at 10
JESTER: Sage and Wizard spells at 3
BARD: Cleric and Wizard spells at 3
METALSMITH: Sage spells at 6
PIRATE: Wizard and Thaumaturge spells at 6
ASSASSIN: Thaumaturge and Necromancer spells at 3