I really don't like that, having her ever having been a true believer would completely destroy the whole dynamic, which is kind of one of the most important elements of the show. The whole idea is that you've got the hardcore true believer who instantly goes to the supernatural explanation instantly for everything, and hardcore scientist who's convinced that there's a scientific explanation for everything who knows nothing about any of the supernatural, and make her an ex-true believer would give her that extra bit of knowledge that would totally change the dynamic.