@BJRGF The Potent Cure Elixir turns any occult sim back into a human, at least among the occult types included in the Supernatural pack. (I haven't tried the others; I do know mermaids become human if they "dry out.") It can be learned at level 7 of alchemy and requires wolfsbane, a glow orb, and a ruby, or you may be able to find one at the elixir shop.
Removing or adding an occult status in CAS can break a sim, though not usually permanently, and the elixir may not work afterwards. So you might need to give your sims elixirs to turn them back into their occult form and then use the potent cure elixir on them afterwards. Or you can use NRaas MasterController to add or remove any occult status with a few clicks.
By the way, when you say you have the complete collection, what exactly do you mean and where did you get it? It's not usually referred to that way.
