From the initial house rule here.
Dual-classing is an option mostly for humans, although demi-humans are eligible when they hit ninth level (Elves to Magic User, Dwarves to Fighter, Hobgoblins to Thief, Halflings to Druid)
At any odd numbered level past first (and no, “aren’t they all odd”), humans can abandon their old class and begin another one. They can only do this once. They have to have a 15v or higher in the Prime Requisite for their new class (Fighter-STR, Magic User-INT, Cleric-WIS, Rogue-DEX, Druid – CHA) to do so.
Here’s how that works. Current XP drops to 0. They retain attack bonuses, hit points, and any spells they already know. They cannot learn new spells from their old class (which is only really an issue for magic users).
Speaking of magic users, they do not gain the ability to cast spells in armor, regardless of old or new class. (Dual-classed Thief/Magic Users are known as “Night Blades”, dual-classed Fight/Magic Users are “Mage blades”). They may wield any weapons eligible for either of their classes, though.
The upside for clerics is that they can already wear any armor and cast spells. The downside is that if they violate their alignment strictures (“lawful”), they lose their ability to cast spells and repel undead. Their alignment strictures prevent them from dual-classing to Thief or Magic User (but not the other way around. Fighter/Clerics are “Paladins”. Yes, they can use the full range of weapons, god help you.
Dual classed Thieves can use Thief abilities is heavier armor than leather. Double the AAC (excluding any magic bonuses), and apply that as a penalty. Chainmail, for instance, is AAC [+4], so trying to sneak in chainmail is at -8. Dual-classed Fighter/Thieves are “Rangers” or “Assassins”, depending on Alignment. Dual-classed Druid/Thieves are “Bards”.
You keep your attack bonus and hit points, until the new level stats are higher than the old ones. Which is clear enough for attack bonus. For Hit points, roll hit dice as normal when you level, and when your result exceeds your hit points, switch to the new total / dice. Also note that at first level in your second class, you roll for hit points normally, you don’t use the maximum possible result (the way you do in your first class).
Hit point example:
- Daffyd the third level fighter has no CON bonus, a DEX of 15 or higher and 16 hit points (the statistical average). Daffyd dual-classed into Rogue. Woo! He rolls 1d4 at first level, but doesn’t beat a 16 (his current hit points). And second level, nope, not better than a 16. At third and fourth level, same deal. At fifth level, he rolls 5d4. If he rolls 16 or less, see you at sixth level. If he rolls 17-20, that’s his new hit point total, and his hit points advances normally as a Rogue going forward.
- He’d be better off, hit point wise, if he had dual-classed from Rogue to Fighter. But then his Rogue skills would be stuck at third level forever.