In my worlds, alignment is not quite as 'strict' as it is under older D&D rules -- there are no alignment languages, and unless I'm forgetting something, it looks like 3E doesn't have them any more either -- but it could just be that I am so used to not having them exist that I just ignore them whenever they are mentioned. Anyways, in my world(s), there is no such thing as alignment languages. Alignment does represent the character's general outlook on life as of the moment it is selected -- but that can certainly change over time. In the real world, for instance, I know people who when younger were wild and crazy guys, drinking and carrying on, yet who later settled down and became ordinary, law-abiding citizens with families -- clearly their 'alignment' changed in some manner, shifting from chaotic towards lawful over time, and possibly from neutral towards good in the other axis. Do they occasionally get nostalgic urges to go back to the 'good old days'? Maybe... Similarly, I have known people who started out trying to toe the line, do what was expected of them, etc., and who for one reason or another slipped away from the ideal, becoming more reclusive or antisocial even. For whatever the reason, people do change!
That said, you are both right -- alignment changes are never trivial, but what Eric was getting at was that 'in game' reasons for alignment change do not have to be incredibly dramatic or difficult. Just as the GM can observe that a character's actions are consistently skewed away (even if just by one step on the 3x3 matrix) from his specified alignment, and let the player know that the character's alignment is either in danger of changing or actually has changed, so too can a player, having played a character though several expeditions, realize that the players conception of the character has changed slightly, or that even if the GM hasn't noticed, he is acting slightly differently than his officially chosen alignment. And this can represent either a slow but consistent change, or potentially (through a dramatic, possibly traumatic, event) a life-changing moment.
It should never be, as Jon says, a situation where the player decides, gee I want to do that but I cannot because my alignment doesn't match, so I'll change my alignment. But Eric was not saying that -- he was implying that a life-changing event might spark a character's alignment change and the adoption of an alignment-based class such as Paladin. For instance, a character might be neutral good, but due to some traumatic event come to the realization that there is indeed a need for order and structure, and make a pledge to devote herself to righting a wrong which happened because she was trying to do good by bending the rules -- and ended up messing up -- and now out of guilt or remorse she seeks to 'mend her ways and make amends'. Such a character could well be shifted, at that moment, to lawful good, and from that point forward the character would be trying to measure up to her new self-imposed standards. This would not be because the player decided he wanted to play a paladin -- but because an event in the character's life caused her to revaluate her reasons for living... The fact that her new alignment then qualifies her for paladin-hood is a side effect, and in pursuit of her new mission, she might well then 'see the light', be born again', or whatever, and begin advancing (when her next level is gained) as a paladin.
In my previous world, there is a fine example of the slower type of change, involving (no coincidence!) Eric's character, Throin. Throin is a Dwarven fighter/thief. His background was essentially that of a guy who, because of his superior physical skills (and growing up on the streets of the largest town on the island campaign area), ended up acting as an enforcer for one of the more effective 'gangs' in the Dwarven quarter of town. He did what he was asked to do, dealing with his specialty (locks and traps) or his other specialty (wielding a pair of daggers with ruthless efficiency), but after a while (i.e. the end of his background and the start of his adventuring life) his gang life just didn't satisfy him any more -- he wanted bigger things -- or at least more valuable things than he was getting as part of the gang! So, as a Neutral character, he began looking for opportunities to adventure. He joined up with a hulking big warrior (Orion's ranger) and helped clear out some vermin of various sorts from some tunnels and chambers under a big mound of earth near a village (which had been being bothered by some giant spiders and such) mostly because he heard there was loot there, according to a couple other adventurers (their previous characters) who had died trying to accomplish the same thing but who were just not tough enough. Anyways, he did get some loot, and fond he liked the adrenalin just as much, and his adventuring career began in earnest. However, over a long period of time, he was much impress by the example set by members of the Temple of Light -- a non-evil generic faith (which believes there are LOTS of deities and other powerful beings which serve forces even greater than themselves, i.e. either Light or Darkness) containing clerics, monks and paladins, and which the bulk of the people belonged to. He was impressed both by the PC example of that faith -- Sister Margaret (Maggie, Dave's character) who was both a healer and a fierce warrior for the Light -- and the members of the faith at temples and shrines throughout the land, for though he had never before paid much attention to religion, he found himself often in need of healing and aid of various sorts, and the minions of the Temple of Light were always willing to help him -- for FREE! This last bit intrigued him severely -- so much so that in spite of his own tendencies, he began of his own free will to donate some of his loot to the temple, attend services, and actually listen to the words of the priests and others... He saw that actually helping others came with its own rewards, even if they could not be counted like coins, and he found himself thinking about the precepts of the faith and making decisions partly based upon such precepts. And, over time, he joined the temple as an actual lay member. Now the temple does not actually require that the members be Good-aligned, just that they not be Evil, but Throin himself could see that the essence of the faith was in its altruistic nature, and he found himself more and more acting in a way that one would call Good... As a character, he never consciously made the decision to become Good -- but Eric and I discussed his changing outlooks and decided that he had, indeed, had a change of alignment over the course of several sessions, and modified the character sheet accordingly. And the next time the character picked up an item that was aligned, the magic reacted as appropriate for his new official alignment, and he shows up on Detect Good.
So that sort of thing can certainly be done -- and if a player does want
to change alignment for some game-mechanics version, well, then it may
require either sheer luck (having an event occur which would justify it,
and then taking advantage of that) or a decent amount of work on the
part of the player, playing the character according to the way he wants
the character to eventually end up. It should have some reasonable
rationale as well, but it can be managed. In the case, hypothetically
speaking
In the case of a character whose existing class has alignment restrictions, though, it would probably have to be a more dramatic change of direction -- Jon's wu jen, for instance, is unlikely to want to stop studying magic in the way he has been trained to do, just because some other way of studying sounds interesting. But say the wu jen was killed (again), and raised from the dead (again), by someone who might be a member of a particular prestige class (still), and the wu jen got into a discussion with the character who raised him, and she turned out not to be a shaman at all, but instead a character who combines knowledge of wu jen spells with knowledge of divine spells and martial arts skills as well, and offered to teach her secrets to the recently raised wu jen, if he thought he could discipline himself enough to learn those secrets, and the wu jen said he was willing to try...
IN this case, in other words, the trauma of death and being raised might cause the character to take a serious look as what he is doing with his life, and why does he keep getting himself killed, and is his current life path the best way to achieve his actual goals, and other questions as well, and perhaps decide that he might indeed want to learn these secrets as offered to him by the lady who had once again brought him back from communing with his ancestors... It would not be required -- but it would be a nexus point at which the player COULD indeed choose to revise the wu jen's alignment and career path. Or he could simply resolve to be more careful -- "this time for sure" -- and try to avoid getting killed once more. Perhaps the character, instead of deciding to follow a new path entirely, might simply decide he needed to exercise and work out more, and learn a few more skills which might help him survive better -- and instead of taking his next level as a wu jen, might take it as an aristocrat (unbeknownst to his companions) and get better hit points and BAB so as to be better able to handle the occasional melee he might get himself into... So the same traumatic event might lead to a significant change, or no change, or simply a slightly different emphasis on advancement based on his existing classes.
In all cases, it is better for any advancement the character does to be supported by role-playing activity, whether it be specific encounters, a gradual change in the character's motivations and decision-making process, or simply a desire to change careers supported by a reason to do so. Never on a sheer whim, but not incredibly difficult either! After all, in the end the intent is for everyone to be having fun while feeling that their character is, to some extent, 'real'.
The GM will clearly give some players slack, based on their level of role-playing ability -- if Laura's character want to add a non-ranger class that would make sense for the character, I wouldn't necessarily require that she play up to that over multiple sessions, but if Pete wanted to proceed in a new direction, it would definitely require some forethought!
Whew! Enough said -- we all want alignment -- character personality etc. -- to mean something and be relatively consistent, but it doesn't have to be unchangeable. And certainly changing multiple steps on the matrix is right out, except under the influence of powerful magic... ;)