This particular set of house rules (untested yet) will undoubtedly be controversial. I know a lot of people in the advanced circles have strong feelings about dual-classing, and it doesn’t even exist in B/X. But it existed (and made a huge campaign impact) in my first campaign, so its important to me that players have this as an option.
You may switch a class anytime you have time to train in a reasonable fashion for the class (often having to leave play for a few months at the least) and pay whatever costs the DM considers reasonable (rarely less than several thousand gold pieces for the compressed, advanced class). You must have at least a 13 in the prerequisite(s) of the class you are switching too. To switch to a new class you must:
1) Earn enough xp to gain a level in your highest levelled class. This class (whatever it is) is called your Primary Class. You must declare your intent immediately upon gaining a level in your current class. So a fifth level fighter wanting to become a 1st level mage would need a total of 32000 xp and they would have had to announce their intentions immediately upon reaching 16000 xp (or as close to that as possible, basically as soon as they cross the threshold). Note, all xp earned to gain that next level is lost (so the character in the above example would end up being a 5th level fighter with 16,000 xp and a 1st level mage with 0 xp, the 16,000 they needed to reach level 6 would ‘disappear’.
2) You then become level 1 in your new class.
3) You gain all the standard abilities of a 1st level character (except general proficiencies and extra human proficiency).
4) Gain hit points as normal for your new class, keeping two (or more) separate totals. Use whichever is permanently higher as your actual hit points.
5) Use whichever individual Save and attack throw is best for the classes at the level they currently are. So a Wizard 5 who picks up 2 levels in Fighter would fight as a 2nd level fighter (9+) but would use all of his wizard saves as they are superior to a 2nd level fighter. If the fighter level increased to 7, they would use the saves for fighter in every category except versus Spells and Wands/Staves, because a 5th level Wizard has a better save so the PC would keep those.
6) Whenever awarded xp, you must decide which class to give it to. Your primary (highest level) class receives xp at full value. Additional classes receive xp at 1/2 . You must allocate all xp earned in an adventure to a single class. You may never advance more than a single level per adventure, regardless of xp earned you will lose anything over 1 shy of earning a second level. A tenth level wizard, 1st level fighter goes on an adventure. He earns 10,000 xp. Grimly, he applies it to his fighter levels. After he divides it in half, he has 5000 xp. Since a fighter needs 4000 to go to level 2, he loses 1001 xp, leaving him with 3999. All totalled, he has ‘squandered’ 6001 xp to learn fighter abilities.
7) If your new class ever reaches a higher level than any previous class, it becomes your Primary Class. Any new classes would be bought according to its experience chart. You would gain full experience when spending XP on it, and half XP on your other classes.
8) Proficiency bonuses (such as Divine Favor) only apply to the class they are gained with. So a Cleric/Fighter would get +2 to all Cleric saves, but not Fighter ones.
9) If a character uses a weapon forbidden by one class, they must use the attack values of the class that allows the weapon to be used. So a Cleric 10, Fighter 1 would fight as a 1st level Fighter with a sword, but a 10th level Cleric with a mace.
10) Arcane magic is limited by class and by armor. So a Fighter/Wizard could NOT cast spells wearing any armor (unless they are an elven spellblade or similar). A character with two classes that have magical abilities (Mage/Venturer) would neither combine their casting slots or their repertoire, but would keep them separate as two distinct styles of magic. If the character wanted to be able to cast a spell using slots from either class, they would have to give up a slot in their repertoire for both classes. Thief abilities are limited when wearing armor heavier than leather, though common sense should prevail (a fighter in plate could Hear Noise if they took their helmet off, and backstab if they were somehow silent or caught an opponent off guard, but they couldn’t climb or move silently or hide in shadows).
11) If you have more than 2 classes, the maximum level of their additional classes is limited. The first additional class can be as high as the primary class. The third and later classes cannot advance to higher than a total number of levels equal to the levels in the Primary Class. So a Fighter who had taken on several classes and advanced to 10th level as a Fighter, could have one class up to 10th level. The others could not, when added together, be higher than 10th level. This is to prevent extremely high level characters from dabbling incessantly rather than advancing their primary skill set(s).
12) Level Drains target the primary class. If you are level drained, all drains continue to target the same class for the remainder of the adventure. If drained to the point that your primary class level is below that of another class, and you survive the adventure, the higher class now becomes your primary class. Restoration rituals can reverse this as normal, and cause the other class to return to being your Primary Class. In other words, having 4 or 5 classes isn’t going to let you survive a vampire attack any more than a single class character would have.