Okay, so their are Clerics and Evil Cleric templates, and Holy and Unholy Warrior templates.
What happens if you want to switch?
Or, well, let's just say your god wants you to switch and encourages you along by damning you to hell and excommunicating you.
Luckily we already know how to make the Evil versions from DF 3 (Evil Clerics are on p. 23, Unholy Warriors on p. 27). But both are written from the perspective of a starting PC. The lenses can be tacked on later, but the base templates are where you start.
We just need to figure out what to change on the character sheet.
- you're now Excommunicated. Good job. Get rid of that holy disadvantage you took - Honesty, Sense of Duty (Coreligionists), or Vow.
- you'd need to swap out the relevant Holy Advantages for Unholy ones. The Evil Cleric and Unholy Warrior have suggestions.
- Do you get to change disadvantages to more evil ones? I'd say change as few as possible if it's an unwilling conversion, as many as you want if you snap and go all Erac's Cousin.
- Skills? Can you swap them out? This one is a real game-world decision. Can you (new or old) god swoop in and change what you know ("My knowledge of healing herbs is gone . . . but I know how to use them to kill!")? If so, you'd have to swap out the "good" stuff for the "bad" stuff. If not, well, you get to keep your helpful skills and you need to seek out a source of evil to learn the bad stuff. Either is made of pure awesome.
- Spells? This is what sparked this whole post in the first place. Good and Evil get different spell lists. You'd either get to:
a) re-pick all of your spells.
b) re-pick the ones that don't match your new list.
Either is plausible. I'd say a), if only because it's vastly easier to do.
One further idea is to simply grab up all the points you put into spells and holy powers, and redistribute them amongst your new powers. You have 23 points in spells and 35 points in holy powers? Pick 58 points in new spells and powers. Or your GM picks some or all, depending on the circumstances.
That's my back-of-the-envelope sketch of what you'd need to change. If I missed anything that needs addressing, let me know.
So if your GM doesn't seem to be generously handing out Holy items, or that black-bladed howling sword seems pretty nice, actually, maybe you just might make the switch . . .