In the Infinity Engine games (and in the Gold Box games that allow it), when a character dual-classes, she loses her old abilities until her level in her new class surpasses the one in her old class. What if this didn't happen?

(Example: Imoen, if dualed into mage, would still function as a thief before gaining enough mage levels.)

Would this break the game, or would it be balanced when compared to multi-class characters? I think it might be balanced in games that cap based on total XP (like the Baldur's Gate games), provided the character isn't using a kit.

It would put humans at an advantage, since they're the only race that can dual class. You are essentially saying they can multi-class into anything, at any time. So yes, I think it would break the game.

Here is some preliminary analysis.

We are here going to assume that the character doesn't have a kit, and the other rules for dual-classing (including those specific to the engine that aren't part of the pencil and paper rules) apply.

Case 1: Early dual class.

In this case, the character dual classes when experience requirements still grow exponentially. This case is not affected that much, as it turns out (assuming the BG1 XP cap doesn't come into play). Thing is that the loss of class abilities lasts only about as long as it takes for the party to gain 1.5 levels or so, perhaps less (especially if you class change into thief/mage/druid during a level range where the target's XP requirements are low). As a result, this change wouldn't have as much effect then. To see this in action. start Baldur's Gate 2 with a level 7 Ranger and dual-class her into Cleric.

Case 2: Late dual class.

This is the case where you are normally sacrificing a lot of power in the short run for more power in the long run. With this change, you no longer have the short term sacrifice; your level 13/1 fighter/mage can still fight as well as a level 13 fighter. A bit later, you have a character who is level 13/9 instead of 14, say. This does lead to a character who might be rather strong. However, you still can't go back to leveling up fighter, and perhaps more significant, the IE games are capped by total XP earned. In other words, there is a long term cost to doing this. (Also, you get fewer HLAs this way. If it's desired to fix this, one would need to grant HLAs everytime the player levels up, has at least 3 million total experience, and (to prevent abuse to get more HLAs) has reached what used to be called "name level".)

Case 3: Very late dual class

This is the case that is currently punished in the Baldur's Gate series. If you dual-class from cleric to mage at level 25, you will never see your character's clerical abilities again. This change would make this setup viable. This setup behaves quite differently than the early dual class case. An early dual class gives you a character who is evenly multiclassed at first but advances further as a single class character. A late (or with this rule change, very late) dual class gives you a character who is essentially single classed early on, but behaves as though multi classed later on. There's also the fact that I like the idea of a character who picks up a new skill late in her adventuring career (and also makes level ups more interesting at that point), and I think the rules should encourage that sort of thing.

This post is getting long, but I have some things to say about human versus demi-human balance, so I will make another post later in this topic about that consideration.

Here's another thing:

Demi-humans have a bunch of advantages. Of particular note are multi-classing, which is easier to use than dual-classing (and also advantageous in ToB, when new levels stop becoming useful and HLAs become a factor), and ability score modifiers. Half-elves are great for multi-classing and are better than humans in every way except the inability to dual-class or be paladins. Half-orcs can start with 19 strength, which is especially nice if you're playing a cleric or thief, as 19 strength is better than any amount of exceptional strength.

Then you look at humans, who have only two advantages: 1. Can be a paladin (but why paladin over fighter/cleric?) 2. Can dual-class

Dual-classing, however, is much harder to do successfully than multi-classing. First, you have a period when you are weak. Second, if you dual-class too late, you are weak for the rest of the game and might as well have been single-classed. Figuring out when too late is requires looking at XP tables and doing the math. My proposed change would get rid of these two issues.

One other thing to note: In the pencil and paper game, demi-humans have level limits, which were in theory meant to balance the advantages they got at lower levels. The Infinity Engine developers chose deliberately not to implement this rule, which makes demi-humans more powerful than humans in the long run. In fact, with the XP cap working the way it does, there really isn't much reason (from a gameplay standpoint) to play a human!

Currently, demi-humans are better early (racial advantages you get at level 1), mid-game (multi-classes get full use of abilities, while dual-classes do not), and late-game (multi-classes continue to level and get HLAs for both classes when single/dual-classed characters tend to stall at this point). My change would actually make humans worth using.

There is no case in 2nd edition AD&D where experience point requirements grow exponentially. You'd choose Paladin for Carsomyr and the ability to play as an Inquisitor, which may as well be called Mage Murderer Magnifique. Unless you were playing solo, you would have no need for the cleric spells, and would get the most important melee HLA anyway, Whirlwind. If you think humans aren't worth playing in BG2, you haven't played a successful Kensai12 (or 9 if you're impatient or playing without ToB) -> Wizard, which is the poster child for breaking AD&D mechanics wide open. I love multiclassed characters, and had a fun playthrough with a cleric/mage CHARNAME and Aerie. Rough start, but by mid-game it was quite the power couple. But there's still nothing I've tried (and I've tried almost everything I've seen suggested as "supreme power builds") that holds a candle to the Kensai -> Wizard. You can solo Kangaxx in under fifteen seconds with no exploiting, for god's sake.

EDIT: typo, of course.

Actually that's outright wrong. If you look at the first 9 or 10 levels of a typical class's XP table, you will notice that each level takes about twice the experience requirement of the previous level. That is, by definition, exponential growth.

The XP requirements of the Fighter, Paladin, Ranger, and Cleric approximately double for every level up through 10th level, at which point the exponential growth stops. For Mages, the common ratio decreases to 1.5 after level 6, but it is still exponential until after level 11. Thief and Bard XP requirements double up through level 7. The druid XP table is weird, but note that level 14 requires twice the XP of level 13, and level 15 requires twice the XP of level 14.

So, yes, XP requirements *do* grow exponentially.

It's worth noting that, for fighter-types and clerics, the XP needed to gain one level after level 9 is enough to bring a new (or newly dual-classed) character up to level 9.

Also, for Kensai->Mage, why level 12 when just one level will get you the final half attack?

If you have to change the function to best-fit, it isn't a single function anymore. Yes, for part of the character growth experience requirements double, but for most classes that's only the first handful of levels. Still, you're right that you can select parts of the exp table and find doubling.

Let's see if my memory holds: warrior and rogue double to 7. Paladin and Wizard to 6. Cleric to 5. Druid to 3. That's a lousy small selection to cherry pick from to see exponential growth.
EDIT: Ahh, forgot bards. And forgot to mention that rangers use paladin table. Aren't bards the same as thieves in 2nd? Improved THAC0, mainly. Weapon speed. It's nice to hit first in the round and never miss.

EDIT again: Oh, and nine or twelve levels instead of one because hit points are always nice. 130hp mage isn't shabby.

Final edit for this post, pinky swear: If you really want to do late-game dual-classing, just play 3rd edition. They basically dealt with the complaints about that there. If you want to do mid-game dual-classing, then you should only have a period of a few minutes where you don't have your old skills. If you go to wizard, drop party members, pause, and scribe scrolls like a madman, then tell them you were just kidding when they walk over and ask if you're sure. If you're going to any other class, just clear up to right before the big quest rewards for a handful of places like the Keep (or all the strongholds if you've got that mod), slaver's den, or Aran Livail/Bodhi. Since you have a post three topics down talkign about how you underflow gems to get gold, you don't ahve any standing to call any of that excessive cheesing XD

Ouch! Very true, though. :)

Dual classing was broken in 2ed D&D, in PnP as well as in BG...

I agree with this idea. At my current playthrough i duel classed imoen at level 4 because i didnt want to play the whole game with a good-for-nothing mage. But now she can use her bow excellent while using magic when needed. But if she wouldnt lose abilities when dualclassing, then i would dual class her at level 6 or 7.