It is rare that a person's beliefs are a bunch of interchangeable modules that can be individually lifted out and replaced.
As a former fundamentalist, I can attest that often a person's entire set of beliefs are intertwined and interconnected, and one cannot simply change one "small part" of her beliefs without ripping huge holes in everything else.
For a Christian literalist, it is not as simple as simply believing that Genesis is literal history and then believing that Genesis is metaphor. To go from one belief to another can be the intellectual and emotional equivalent of a stroke; one is left having to learn how to move all over again.
"Intellectually, scientifically, even artistically, fundamentalism -- biblical literalism -- is a road to nowhere, because it insists on fidelity to revealed truths that are not true." -- Katha Pollitt