What the rules say
The Player's Handbook II on Class Level Rebuilding with regard to prestige classes if a creature already has levels in a prestige class is unclear:
You can also use this method [i.e. Class Level Rebuilding] to trade out (or add in) prestige class levels, though if you want to take levels in a prestige class that’s new to your character, you must be able to demonstrate that he can still qualify for it [the prestige class] using what he has gained from his remaining class levels. (197)
(Emphasis mine.) "But," you may ask, "what if I want to take levels in a prestige class that's not new to my character?" For example, can a wizard 3/cleric 3/mystic theurge 1 that completes a rebuild quest pick to lose one level of cleric to gain one level of mystic theurge? The rules-as-written are silent.1 Ask the DM.
What this DM would allow
This reader's gut initially said No, a creature can't bootstrap itself this way if for no other reason than, compared to the remainder of the game, that result seemed too generous. I mean, no other effect allows a creature to trade class levels it doesn't want for ones it wants. No other effect allows a creature to transform into an amalgam of class levels that otherwise could not exist.2 Then I kept reading. Before we can even appraoch this question, we have to take a lot of things into consideration:
- Does the DM allow class level rebuilding? I know that sounds like a no-brainer, but a player shouldn't assume these rules are being used. Even a DM that tries to use everything will still find some things beyond the pale. (For example, this DM's campaigns don't allow rebuilding yet do allow retraining.3)
- Is the DM willing to develop the rebuild quest? The DM must be willing to design a rebuild quest or use an existing one, perhaps reskinning it for the DM's campaign. A DM may not have time to create a lone PC's adventure, existing rebuild quests may not apply, and reskinning may still yield an inappropriate quest. In other words, a DM may nix a rebuild quest request because it's a hassle.
- Reminder: A rebuild quest should be specific. Unless the DM has PCs that are all headed in the same metaphorical direction—they're all ex-paladins wanting to return to the fold, for example, or they're all sinister wizards looking for necromantic know-how—a rebuild quest will only benefit select PCs. The example quests make it clear that there's no general quest a PC goes on to rebuild but, instead, that there are quests that reward those who complete them with the ability to rebuild within specific guidelines.
- Is the DM willing to allocate time at the table to the rebuild quest? The quest is not supposed to occur off-screen but, instead, during the course of the regular game. While the example rebuild quests seem fairly short (looking to this DM as if they'd take maybe one to four 4-hour sessions), the DM must be willing to pause the larger campaign until the quest's completed.
- Will the PCs successfully complete the rebuild quest? Again, while this sounds like a no-brainer, the assumption seems to be that the rebuild quest will be completed successfully. However, the example rebuild quests are quite challenging for typical PCs that aren't particularly optimized. Examples seem to indicate that PCs who go on a rebuild quest have a significant chance of failing to complete it.
- Will PCs be able to go on another rebuild quest? The PH2 says that rebuild quests are supposed to be rare and that a PC probably won't go on more than a couple—if any!—during the campaign. In other words, using the Class Level Rebuilding rules is likely a once-per-PC-per-campaign opportunity. Use it wisely.
To sum up, this question arises only if your DM approves the class level rebuilding rules; then your DM develops a rebuild quest specific to your desires for your character; then everybody spends table-time going on your character's rebuild quest; then, after your character successfully completes his rebuild quest—that is, by the way, in all likelihood, his only rebuild quest ever—, you ask the DM if your PC can trade one level of wizard for one level of Nar demonbinder.
After all that, this DM says, "Go ahead! Have fun!"
But just in case some kind of further tacit validation from the game itself is absolutely necessary, the Player's Handbook II in its introduction to the chapter "Rebuilding Your Character" says
After your character goes through the retraining or rebuilding process, you might notice that he doesn’t quite match the specs of a similar character built up to the same level by the normal method. Maybe his skill points don’t add up quite right, or his hit points are off a bit from the expected value. But the small variations that crop up in this process don’t significantly impact play balance, and writing rules to eliminate them would complicate the process without really improving the quality of your game. (191)
…And while this section's examples are things like skill points and hp, this DM is broad-minded enough to include among those "small variations" an impossible collection of class levels that couldn't be attained absent the rules on Class Level Rebuilding, especially given the time and trouble necessary to use such rules in the first place.
So, in the end, honestly? If the new configuration isn't game-breaking then whatever. You get through all the Class Level Rebuilding rigmarole then I figure you've earned being able to play your otherwise impossible wizard 6/Nar demonbinder 2. Good luck explaining how that happened on a messageboard or to your next DM, though.
1 Pure Idle Speculation: This reader thinks this problem might've arisen during the book's editing. The clause if you want to take levels in a prestige class that’s new to your character could've been originally if you want to take levels in a prestige class that are new to your character, which is technically borderline acceptable but on a quick read looks really clumsy and wrong. (That is, without the prepositional phrase the clause becomes a more comprehensible if you want to take levels
in a prestige class that are new to your character.) In other words, perhaps there'd've been an ugly sentence but no ambiguity if the book's budget allowed an extra $0.05 for an are. (So you know, this editor would've changed the original's word order so that it read if you want to take prestige class levels that are new to your character… which eliminates ambiguity and saves that precious $0.05, but I have the benefit of hindsight, I'm not on a deadline, and nobody asked me.)
2 The core rules are vague on what levels are lost when a creature's been level drained (although the FAQ less so). (For details see this question.) Gaming level loss—while always dangerous and usually expensive—may be the alternative that you're looking for to yield an otherwise unbuildable character. Again, though, ask the DM.
3 The retraining rules, I must say, have worked wonderfully well in the two campaigns in which I'm using them. Both campaigns involve mainly Tier-3-or-lower PCs, and retraining has made them much more interesting.