When someone dies from Ghoul fever, and they are a 6th level fighter, do they turn in to a Ghast with 6 levels of fighter? Or do they just become a 6HD ghast? I know the answer to this in D&D 3.5, but unsure if it has changed in Pathfinder.
They're not a fighter any longer
According to PF SRD, on both the ghoul and ghast description of the fever:
Therefore unless other spells or foul play are in effect they would be the basic form.