Shaman becomes Witch?

So I have a player who started out as a Shaman, but did some bad things that caused an aligment violation (poisoning the town werll in The Keep!). I took away all his magical powers. He atoned, but now he wants to change classes to Witch (the one that can control Undead; I think it's called Vouduon.or something like that). He's currently fourth level. Any suggestions on how to make this a smooth transition?

Shamans and witches are both divine casters and both can have links to ancestral spirits or animal spirits.  Have the PC try to contact his/her ancestors (as per the shaman ritual and have it result in the defining of a new relationship with a particular ancestor.  That ancestor becomes her new spirit guide and teacher and the PC becomes a witch.

If you want to formalize it in the game, have the PC train to get a new proficiency (soothsaying, say).