Deset Gled said:
Note that the rule from CW does not appear in the core rules or the SRD, and is incompatible with the Dragon Disciple PRC (which is core). I consider it to be an optional variant, like all other material from splat books.
It also isn't consistent with the effect on alignment changes on some core classes. I'd say it's a judgment call for the DM. (If I was the player, it would also be an academic one to make with his brand new NPC, but thats more a general forum discussion.

) I would consider whether the alignment restriction is based on divine investment or willingness to train.
The Assassin alignment restriction, for instance, I see entirely as a willingness to train issue, esp with the "Kill someone just to get into the class" requirement. There is a order of assassins who will only train the evil, so you have to be evil to be trained. If an assassin has a change of heart and becomes neutral or good, they are no longer welcome by the order and thus cannot be further trained, but the idea that they would magicly lose their training is a bit wacky.
On the other hand, a paladin-esq PrC where the majority of class features are gained by divine power harnessing the cosmological force of Good is not going to keep getting those powers channeled when her alignment changes. Both of the PrCs in this example sound like they are going to fit this classifications more than the assassin one, and the majority of class features (definitely all spell, spell like or supernatural ones) should be lost.
The Barbarian class, while core and not PrC is an interesting cross, in that a lawful barbarian losses Rage and the ability to progress but not other abilities. Rage, while not divine, does make sense to be related to a specific mindset rather than just something you needed that mindset to train in.
So, I think the one size fits all CW rule is a bad one and it's good that it can be considered optional. Look logically at the PrC and whether the alignment allowed the character to train in their abilities or actually powers the abilities at some level. Then go from there.