The rule I'm likely to adopt is "Career Completion is Mandatory. You must finish your current career before swapping to a new one. You may have an exception to this rule only once in the lifetime of any given character."
The point of the exception, is to allow for storylines and character development situations such as:
- A dwarf is terribly dishonored and feels he has no choice but to seek glorious death as a Troll-Slayer.
- A character saves the life of a major noble and is immediately Knighted for their valiant heroism.
- War breaks out and the players volunteer as soldiers to head to the front lines.
- A previously law-abiding character turns to crime to avenge a misdeed or defeat a bitter rival.
- A wizard dabbles in forbidden magicks and ends up cast out by their order.
- A character marries into a noble family, and has to start living up to Gold Tier expectations.
- A player grows tired or dissatisfied with their character, and thinks a major change of focus will refresh their flagging interest in playing that character.
The reason for the rule itself, and for limiting the exception to once-per-character, is because there's a glaring rules exploit possible if you're willing to swap careers willy-nilly to cherry-pick specific advancements. Especially for humans.
The most notable such exploit would be piling up Fortune Dice on your Characteristics. In and of themselves, Fortune Dice are not that amazing. On any given roll, each one has a better than 50% chance of being useless (by either rolling a blank side, or rolling a symbol you already had more than enough of). Compared to the Expertise Dice that come with Skills, Fortune Dice are very weak. One Yellow Expertise Die is a little better than 2 White Fortune dice. However, Fortune Dice are cheap and the only thing restricting their purchase is tied to your career. Skill dice, on the other hand, are restricted by both your career and your rank.
The default assumption of the system is that in most cases you'll complete your career before moving on. If that's what you do, in about 12 XP you'll add roughly 1 Yellow Die and 2 to 3 White Dice to the dice pool of your best action.
The cherry-picking approach where you never complete a career, and just continuously cycle to (and abandon) compatible careers that allow for Fortune Die accumulation could result in anywhere from 7 to 12 White dice in those same 12 XP.
This could quickly break the game. After 2 ranks of this, a PC using such an exploit would be rolling 20 to 30 dice per action in their area of focus, with success rates over 99% on their most likely actions. This would lead to a ridiculous "arms race" with the GM and their fellow players.
And it's not the Fortune Dice that are truly the problem, they're just the most likely manifestation of the problem. The rest of this post details other things that get broken by the cherrypicking and career-swapping approach.
If a character decided to just max out Wounds, they'd have even more freedom in terms of which careers to cherry-pick from. It would be like a D&D character deciding that instead of leveling up, they were just going to accumulate hit points at 4 to 6 times the rate of any other PC. That'd be a little bit broken, but mostly just really boring.
Actions, Talents, Skills and Stance Pieces have other things that keep them in check, but at the very least being able to swap careers temporarily undoes whatever soft limits are imposed by the "Advances" box on your career sheet. It doesn't matter that my current career focuses on Skills and Talents instead of Actions, if I can play at being some other career for two or three sessions and cherrypick the Actions.
The Adaptable special ability for being a Human of the Reikland quickly breaks under this rampant career swapping. At character creation, non-humans come out slightly ahead of humans. Humans are basically built with 26 points worth of stuff, whereas Dwarves and Elves are built with 29 points of stuff (though 9 of those points are automatically invested in very specific racial abilities). This is roughly balanced by humans having more careers to choose from, and humans getting a 1 point break each time they change careers. A typical campaign lasts for 3 to 5 ranks. If you're completing careers at the rate of one per rank, means that by campaign end humans have broke even, or maybe come out a couple XP ahead. If instead you're using the swap-and-cherrypick technique, your XP saving breaks even in Rank 1, and surpasses the demihumans at the start of Rank 2. Even worse, this XP break is what makes the cherrypicking even viable in the first place. Humans can accumulate White Dice (or whatever) at ridiculous rates, but non-humans would struggle to get any significant advantage out of the exploit.
Aside from all the game-breaking mechanical impacts, the worst part of this exploit is what it means for the setting.
If anything, one of the flaws of the default career system (even with my house rule in place to shore up the exploit) is that it really pushes character advancement and progression along at a breakneck speed. You're basically guaranteed to "get a promotion at work" every 10th session. When sessions involve long passages of time, that's reasonable. I'm currently running The Enemy Within, which is a lengthy investigative scenario. Because of all the time spent chasing down plot leads, we're basically advancing 1 day of the in-character calendar per session. Most of the logical career "paths" are only 3 or 4 steps (20 to 40 sessions) long. So you could go from squire to knight to head of your knightly order in as little as 30 days. That's already kinda pushing at the believability envelope.
Now imagine what those career transitions look like if you're cherrypicking. (Each day spent is a Fortune Die gained, or an XP spent on career transition.) I spend three days as an Apprentice Wizard. Then I become a Gypsy Mystic fortuneteller for four days. Then I pull teeth as a Barber-Surgeon for two days. Finally, I take a job defending the King's highway as a Road Warden for one day. That's not a character arc, so much as a manic episode. I thought changing careers every 10 days was weird enough. That character, having cherrypicked fortune dice for their Intelligence stat, would actually be much better at spellcasting than their colleague who stayed in the dorm studying as an Apprentice Wizard for 10 days.