In my game group there is a player who plays a warlock-druid (1-1). He started as a warlock. He realized he was playing as an NG, but he took CN for the restrictions regarding the alignment of his first class. The manual says that a warlock must be either chaotic or evil. A Neutral Good does not respect either of the two cases and we wondered what could happen, considering that the ex-warlock does not exist and the choice of alignment is not free

In the Complete Arcane there is written:

Alignment: Warlocks are often chaotic or evil (and more than a few are
both). The powers they serve can be cruel, capricious, and wild,
unbound by conventional views of right and wrong. However, even
warlocks who derive their powers from the most sinister of patrons
have been known to turn the black powers at their command against
evil. A good-aligned warlock is a grim and fearsome enemy of evil. All
too familiar with the darkness lurking in his heart, he gazes
unflinchingly on the evil in others and battles the foulest of foes
without fear.

And at page 7 we can read:

Warlocks have the following game statistics.

[...]

Alignment: any evil or any chaotic

However, the manual does not refer to ex-warlocks for changed alignment
or warlocks that no longer respect the binding alignment and change it.

What happens in these cases?

If a class punishes alignment change, the class's description explains that punishment

For example, the Player's Handbook on Ex-barbarians says

A barbarian who becomes lawful loses the ability to rage and cannot gain more levels as a barbarian. He retains all the other benefits of the class (damage reduction, fast movement, trap sense, and uncanny dodge). (26)

Similar sections exist for the bard, monk, and paladin. However, there's no section in Complete Arcane for ex-warlocks, so while a beginning warlock must enter play with an alignment that's chaotic or evil or—y'know—both, the warlock need not stay that alignment and loses nothing if he changes it, whether that change occurs due to his actions and DM fiat or mechanically like through a spell like atonement. Another class with an alignment mandate but no restriction on changing it later is the Heroes of Horror standard class dread necromancer.

However, with all this in mind, this player strongly recommends talking to the DM about waiving these kinds of "soft" alignment requirements—if not all alignment requirements!—for a PC whose player wants to play against what the game thinks is normal for a particular class.

The FAQ says they can't gain further levels, but that's not stated in any rule.

According to the D&D 3.5 FAQ, page 28:

What happens to a warlock who changes his alignment to LG, LN, NG, or N?

A warlock who isn't evil or chaotic can't gain any further levels as a warlock, but doesn't lose any class features or suffer any other penalty.

However, there's literally no game rule to support this. There's no general rule which says a character cannot advance in a base class if they don't meet the alignment any more—as an earlier answer points out, many classes have their own specific "Ex-<classname>" rule for alignment shifts, but the warlock has none.

This may be an accidental omission, but the errata does not change this. In other words, the FAQ is wrong—it's stating what the rule logically should have been, but not what it actually says in the book. In other words, rules-as-written, warlocks suffer no penalties for changing alignment.

Even prestige classes don't need to continue to meet alignment requirements to gain levels or retain their abilities, according to DMG p.176, although Complete Arcane p.17 does introduce this restriction. Even so, that's only for prestige classes, not base classes, and wouldn't apply to the warlock.

I am not aware of any official rules in 3.5 regarding warlocks of other alignments. I believe Pathfinder does have some, so there might be some useful guidance to be found there.

However, despite the lack of official rules, this is a great roleplaying opportunity.

I would suggest either working in the change of alignment into the back story of the character, and now you have ready made plot hooks: jilted former friends, acquaintances, or enemies who don't believe in the change - both good and evil... not to mention how the former source of power might react to the betrayal, or followers of said being or beings.

Or, even better yet, if it works with your campaign, role play the change of alignment out in real time, and let the character try to escape the consequences of their act.

Even if this isn't the main part of the campaign, it would make for a great side plot or occasional plot twist.

Points to consider:

• warlocks draw their power from a source, usually chaotic or evil, and usually from an outsider, per the flavor text. Traditional thoughts on this concept included making a Pact with Evil, or similar. This contract, pact, or relationship will likely be betrayed during the appointment change.




• an alternate power source is in order, one the previous one is no longer available. This could be a different type of outsider, or perhaps an Elemental Lord, or even an ancient Dragon or immortal Fey. Negotiation with that individual would make for done fun and interesting situations, especially if amusing restrictions or requirements are placed on the character.




• consequences of the character's change in alignment will affect all their relationships: private and personal, public and social, secret contacts, religious, political, business and so forth.




• the character history will still come back to bite them, and past actions and affiliations will often still be assumed to be true, and people will jump to conclusions, make assumptions, and accusations; or even try to follow up on the past deeds of not only the character, but also all other warlocks and anyone the warlock used to know... even things completely unrelated will be accused against the character.

These are just a few possible angles that could add depth and spice to the character, the party, and the campaign.

I think this is an issue of framing and preconceptions.

I don't think there should be any penalty, except for the eventual comeuppance associated with leaving a faction. I generally think of Patron powers as more of a "Tap" into the greater power which is the patron itself. Even if you break the bond, you can still tap that powersource, and even learn to drain more power from it. This inevitably concludes with the PC consuming their patron to reach the max-level.

That is to say, the preconception is that this power is a constant and willing gift of the patron. If you instead frame it as a one-time gift of the patron, then you can do or be whatever you want after you have it.
An Ex-cultist, a Prisoner who was possessed by a demon, or just a dumb kid who made a deal with the devil are all valid Warlock characters who would have unholy powers but not an association with their patron.

Whenever I have a player playing a warlock, I usually have them start with the bond between themselves and their patron broken. (Usually as the end of their backstory and the start of their adventures) I think players do naturally want to play Warlocks with chaotic evil tenancies, which is generally havoc for group cohesion. Chaotic-Evil characters also tend not to bond with the world and become hard to motivate.

I hope this helps.