Clerics Page 20:
"The cleric can be of any alignment (q.v.) save (true) neutral (see Druid hereafter) alignment, depending upon that of the deity the cleric serves."
Druid Page 20:
"They are the only absolute neutrals (see ALIGNMENT), viewing good and evil, law and chaos, as balancing forces of nature which are necessary for the continuation of all things."
Paladins Page 22:
"A paladin character is a fighter sub-class, but unlike normal fighters, all paladins must begin as lawful good in alignment (q.v.) and always remain lawful good or absolutely lose all of the special powers which are given to them."
and
"Law and good deeds are the meat and drink of paladins. If they ever knowingly perform an act which is chaotic in nature, they must seek a high level (7th or above) cleric of lawful good alignment, confess their sin, and do penance as prescribed by the cleric. If a paladin should ever knowingly and willingly perform an evil act, he or she loses the status of paladinhood immediately and irrevocably. All benefits are then lost, and no deed or magic can restore the character to paladinhood; he or she is everafter a fighter."
Rangers Page 24
"All rangers must be of good alignment (q.v.), although they can be lawful, chaotic, or neutral otherwise."
and
"Any change to non-good alignment immediately strips the ranger of all benefits, and the character becomes a fighter, with eight-sided hit dice, everafter, and can never regain ranger status."
Thieves Page 27
"All thieves are neutral or evil, although they can be neutral good (rarely), and of lawful or chaotic nature. Most thieves tend towards evil."
Assassins Page 28
"Assassins are evil in alignment (perforce, as the killing of humans and other intelligent life forms for the purpose of profit is basically held to be the antithesis of weal). They can, of course, be neutral as regards lawful and chaotic evil."
Monk Page 30:
"Therefore they must always be lawful in alignment, although they can be evil, good, or neutral with respect to their approach to lawfulness. A monk who for any reason loses this lawful alignment loses all monk abilities and must begin again as a first level character."
Bard Page 117:
"They must always remain neutral, but can be chaotic, evil, good or lawful neutral if they wish."
Changing Alignment Page 34:
"Changing Alignment
While involuntary change of alignment is quite possible, it is very difficult for a character to voluntarily switch from one to another, except within limited areas. Evil alignment can be varied along the like axis. The neutral character can opt for some more specific alignment. Your referee will probably require certain stringent sacrifices and appropriate acts — possibly a quest, as well — for any other voluntary alignment change. In fact, even axial change within evil or good, or radial movement from neutrality may require strong proofs of various sorts.
Further voluntary change will be even more difficult. Changing back to a forsaken alignment is next to impossible on a voluntary basis. Even involuntary drift will bring the necessity of great penance."
DMG Graphing Alignment Page 24:
"It is of importance to keep track of player character behavior with respect to their professed alignment. Actions do speak far more eloquently than professions, and each activity of a player character should reflect his or her alignment. If a professed lawful evil character is consistently seeking to be helpful and is respecting the lesser creatures, he or she is certainly tending towards good, while if he or she ignores regulations and consistent behavior the trend is towards chaotic alignment (see PLAYERS HANDBOOK, APPENDIX III, CHARACTER ALIGNMENT GRAPH). Such drift should be noted by you, and when it takes the individual into a new alignment area, you should then inform the player that his or her character has changed alignment (see CHANGING ALIGNMENT). It is quite possible for a character to drift around in an alignment area, making only small shifts due to behavior. However, any major action which is out of alignment character will cause a major shift to the alignment which is directly in line with the action, i.e., if a lawful evil character defies the law in order to aid the cause (express or implied) of chaotic good, he or she will be either lawful neutral or chaotic neutral, depending on the factors involved in the action.
It is of utmost importance to keep rigid control of alignment behavior with respect to such characters as serve deities who will accept only certain alignments, those who are paladins, those with evil familiars, and so on. Part of the role they have accepted requires a set behavior mode, and its benefits are balanced by this. Therefore, failure to demand strict adherence to alignment behavior is to allow a game abuse.
Lawful good characters should not be allowed to ignore unlawful or shady actions by “looking the other way”. If, for example, a party that includes a paladin decides to use poison on a monster that they know is ahead, the DM shouldn’t let the paladin be distracted or “led away for a few rounds” when it is patently obvious that the paladin heard the plan. If the player does not take appropriate measures to prevent the action, the DM should warn the paladin that his lack of action will constitute a voluntary alignment change and then let the chips fall where they may!"
Then Changing Alignment Page 25:
"Whether or not the character actively professes some deity, he or she will have an alignment and serve one or more deities of this general alignment indirectly and unbeknownst to the character. Changing of alignment is a serious matter, although some players would have their characters change alignment as often as they change socks. Not so!
First, change of alignment for clerics can be very serious, as it might cause a change of deity. (See DAY-TO-DAY ACQUISITION OF CLERIC SPELLS.) If a druid changes his or her alignment — that is, becomes other than neutral — then he or she is no longer a druid at all! Change of alignment will have an adverse effect on any class of character if he or she is above the 2nd level.
Immediately upon alignment change actually occurring, the character concerned will lose one level of experience, dropping experience points to take him or her to the very beginning of the next lower level, losing the hit die and/or hit points, and all abilities which accrued to him or her with the lost level. If the alignment change is involuntary (such as that caused by a powerful magic, a curse, etc.), then the character can regain all of the losses (level, hit die, etc.) upon returning to his or her former alignment as soon as is possible and after making atonement through a cleric of the same alignment — and sacrificing treasure which has a value of not less than 10,000 g.p. per level of experience of the character. The sacrificial amount is variable, so use your best judgment as to the total and what and where it should go — magic items to build up the NPC cleric, money out of the campaign, magic items out of the campaign, etc. Similarly, such atonement and sacrifice can be accomplished by a quest. Note that, in all likelihood, the character will desire to retain the new alignment, and it is incumbent upon you as DM to ensure that the player acts accordingly. Some equally powerful means (divine intervention, remove curse, etc.) must be used to restore the original alignment before atonement can begin.
Characters who knowingly or unknowingly change alignment through forethought or actions permanently lose the experience points and level due to disfavor. They must also accept a severe disability in alignment language during a one level transitional period. Until the character has again achieved his or her former level of experience held prior to change of alignment, he or she will not be able to converse in the former alignment’s tongue nor will anything but the rudest signaling be possible in the new alignment language. (See ALIGNMENT LANGUAGE.) Although it is possible for a character to allow himself or herself to be blown by the winds as far as alignment is concerned, he or she will pay a penalty which will effectively damn the character to oblivion.
A glance at the alignment chart will show that radical alignment change is impossible without magical means. If one is chaotic good, it is possible to change to neutral good or chaotic neutral only, depending upon desire and/or actions. From the absolute neutral alignment one can only move to some neutral-based alignment. This represents the fact that the character must divorce himself or herself from certain precepts and views and wholeheartedly embrace another set of values, and human nature is such that without radical personality alteration (such as caused by insanity or magic in the case of this game) such transition must be gradual.
It is assumed that the character’s initial alignment has been his or hers for a considerable period prior to the character’s emergence as an adventurer. This ethos will not be lightly changed by a stable, rational individual. It is recommended that you do not inform players of the penalty which will occur with alignment change, so that those who seek to use alignment as a means of furthering their own interests by conveniently swapping one for another when they deem the time is ripe will find that they have, instead, paid a stern price for fickleness."
So Paladins and Rangers become fighters. Monks become new first level characters. Druids become not druids at all (but what do they become? clerics, 1st level characters, or other) and not specified for thieves, assassins, or bards.