On-the-fly Multiclassing Rules for BECMI / BX

Improved Hit Dice

If the new class has a higher hit die than the old class, the player should replace the rolled hit points with a roll of the new hit die. However, since most players don't track the hit points rolled at each level, they can instead opt to add the difference in average expected results, using the following table:

If using the option d10 / d12 HD option from Breeyark, the pattern (half the difference in sides) holds; upgrading from a d4 to a d12 adds 4hp/level.

These extra hit points are only added when a level of a new class with a higher HD is being purchased; once the character's levels match, the new higher HD is rolled as normal.

Multiple Saving Throws

In most cases, the player should choose which class's saving throws to use when creating the multiclass. In the fighter/cleric example, I assumed the new class would keep the fighter saves. Optionally, he could have used the cleric's saves, which are mostly better, though occasionally worse, for slightly more xp (2800 base). If choosing a more expensive set of saving throws, the PC keeps any saves from the old class that were better than the new class, until advancement in the new class provides better saving throws.

For example, when the 5th-level fighter decides to gain levels of cleric, his saves are 10/11/12/13/14. If he opts to pay the extra XP for cleric saves, the fighter saves are better until he reaches 5th-level in cleric. At that point, he takes the best save for each class, or 9/10/12/13/13 (where a normal cleric of level 5 would have 9/10/12/14/13). Then his saves continue improving as a cleric.

Overlapping Spell Levels / Level-based abilities

If our example fighter/cleric reaches 9th level and decides to become a paladin, he'll be gaining cleric spells from two sources, as well as gaining the turn undead feature from two sources. There are several options worth exploring, but it's unclear which is the most balanced:

1. Ignore overlaps: Ignore all but the best source of spells / class feature. In the case of a paladin/cleric, the paladin still contributes detect evil and hirelings, but doesn't provide cleric spells or turn undead.
2. Allow overlaps where possible: Keep the best source, add extra memorization/castings per day from the other source, to a maximum of 9 in each level. A paladin/cleric will get his level in cleric spells, but also his level / 3 in additional cleric spells. For a 9th-level paladin/cleric, this just means an extra 2 first-level spells per day. Around level 21, the 9-per-day limit starts to take effect for level 1 spells. This is probably relatively balanced. Because turn undead has no daily limit, this option ignores the paladin's turn undead contribution.
3. Add sources together: Take the best source of spells / class features, and for each other source, add levels equal to that source divided by the number of sources. So our 9th-level paladin/cleric will cast spells or turn undead like a 10th-level cleric (9 from cleric, (9/3)/2 for paladin). At level 12, he'll cast spells like a 14th-level cleric. Around name level this isn't too bad (since it'll be adding 1-2 cleric levels but the PC has earned almost 1-2 levels of fighter in XP) This is probably unbalanced at much higher levels, however.

Demihuman Limits

The class calculator makes it trivially easy to remove level limits from demihuman classes. The Breeyark calculator suggests this adds 100xp to the base amount. The DM should use caution when allowing this option, in particular if Magic-user spells are involved in the class. The class feature of casting magic spells should be worth a lot more if the class can reach level 36, compared to the vanilla elf's level limit of 10. This accounts for the main discrepancy in the Breeyark calculator, which thinks elves should have a base xp of 4100 (vs 4000) and magic-users should have a base xp of 2050 (vs 2500!). Having unlimited access to magic-user spells is probably worth closer to 2000 base xp, instead of the calculator's 1600.

A similar adjustment might need to be made for demihuman clerics with no level limit.

It's also possible for a demihuman PC to reach maximum level and then decide to start spending that extra XP to remove the level limit; this will end up costing close to 50% more xp per level than just starting with that option, so it's not absurdly unbalanced.

Another consideration is demihuman hit dice. The class calculator also makes it easy to change the hit dice or saving throws of a demihuman. For instance, it's possible to make an elven fighter/magic-user with a d8 hit die instead of a d6. It's up to the DM whether to allow this; it may make sense to require demihumans to stick to the hit die, saves and to-hit chart of the original vanilla class.

Some More Examples

Some common 1e multiclass options include dwarven fighter/thieves and elven fighter/mages. Gnomish wizard/thief is also a textbook example in 1e, but we'd need to start by inventing the gnome class.

A dwarven fighter/thief has 2850xp/level, assuming the demihuman to-hit chart and weapon mastery, dwarven saving throws, all armor and dwarven-style limits on weapons. The DM might choose to penalize the use of some thief skills in heavier armor, but there's no existing rules for it yet. If the PC started as a vanilla dwarf and just wanted to add in thief skills, the chart up through level 5 looks like:

To extend the chart, the Diff column is New - Old for the next row down.

An elven fighter/mage played this way must have started as a custom class to begin with, before mixing in the other class. The new XP chart is the vanilla elf, but assuming the elf started as a fighter, with a base XP of 2200 (using Elf saves, Demihuman to-hit, fighter specials, elven racial abilities, name level limit).

This is a very expensive way to advance to a textbook elf, but the survivability of a fast-advancing fighting elf might be worth the extra cost in XP.