Are there any rules for a Cleric to change his faith?

Question

There are rules for a Cleric who loses faith, and how to regain his standing through atonement.

But what if he abandons one faith entirely for another? Presumably his new god/faith should be picking up where the other leaves off, so spells aren’t interrupted, and further he probably should be changing his Domains to ones used by his new faith, but does any official book actually describe this process? Does his new faith require atonement for his prior faith in another religion? Is there some kind of interruption time period, where he doesn’t get spells? Is the actual process of substituting Domains explained?

Because the lack of rules that I’m looking at almost seems to imply that a Cleric cannot change faiths.

I’m really looking for book citations here, because this is specifically for the sake of determining what the rules currently are to better judge how changes might affect them. I have plenty of my own ideas about how this sort of thing should be handled, most of which rely on eliminating as much of alignment’s mechanical impact as possible, but that’s not really relevant here.

If your answer is that there are no such rules, I’d like an idea of how certain you are of this. For example, I could not find any such rules in Player’s Handbook or Complete Divine, the first two places I looked. Knowing where the rules aren’t is useful.

Answer 1

Player's Handbook II (3.5) / Divine Conversion / p193

This is a sidebar at the bottom of the referenced page, which is part of a section on rules for retraining. If you don't have the book, here is the direct quote:

DIVINE CONVERSION

As noted in the Player’s Handbook, a cleric who grossly violates the code of conduct imposed by his deity loses all spells and class features and cannot attain any more levels as a cleric of that deity. All these penalties remain in effect until he atones. But what if he doesn’t want to atone? What if a cleric of Hextor finds new meaning and purpose in serving Heironeous after a dramatic conversion experience? Such a character need not become a multiclass ex-cleric of Hextor/cleric of Heironeous. Instead, Heironeous can simply reinstate the character’s cleric powers once he has proven his loyalty, talent, and ability.

A cleric who changes his patron deity must complete a quest to prove his devotion to his new patron. The nature of the quest depends on the deity, and it always clearly reflects the deity’s alignment as well as his or her goals and beliefs. To start the process, the cleric must voluntarily accept a geas/quest spell cast by a higher-level cleric of his new deity. During the quest, the cleric has no access to spells or cleric class features—except his weapon and armor proficiencies, which he does not forfeit.

Upon completing the quest, the cleric receives the benefit of an atonement spell from a cleric of the new deity. The character then becomes a cleric of the new deity and is inducted into the clergy during an appropriate ceremony of the DM’s choosing. After selecting two of the new deity’s domains in lieu of his old ones, the character has all the powers and abilities of his previous cleric level, plus the granted powers of his new domains.

This method is the only one by which a cleric can change his deity. The retraining rules can’t be used to accomplish this task—it is simply too substantial a change in the character’s identity (not to mention his source of power) to chalk up to a bit of practice in his off hours.

This answer essentially reflects the ruling already found in the Forgotten Realms Campaign Setting, but it may be useful to know it's also located in a setting-independent rulebook, just in case there are any sticklers out there.

Answer 2

There is no such rule in the setting-independent supplements that I know.

But setting-specific supplements are another story.

Forgotten Realms Campaign Setting, p233 or something:

Changing Deities

It is possible for a cleric, druid, paladin, or spellcasting ranger (or any other divine spellcaster) to abandon his chosen deity and take up the faith of another deity. In doing so, the divine spellcaster loses all class features of the abandoned deity. To progress as a divine spellcaster of another faith, the character must go on a quest for his new church (often the recovery of a lost item of some importance to the deity), then receive an atonement spell from a representative of his new faith. Once these two conditions are met,the character becomes a divine spellcaster of the new deity, and if a cleric,he chooses two domains from the new deity's repertoire. The character then resumes the class features lost from leaving the old faith(so long as they are still applicable — turning or rebuking undead ability might change, for instance)

While not worded ideally, these rules implement a simple idea: the same atonement spell does the job.

Answer 3

There is another way that isn't mentioned in any of the other posts. A new deity could literally just take on the role of patron of the cleric, especially if it isn't a big leap in portfolios. There is an example of this is the sourcebook Exemplars of Evil (Calais Archwinter).