Catalyzes the epimerization of the S- and R-forms of NAD(P)HX, a damaged form of NAD(P)H that is a result of enzymatic or heat-dependent hydration. This is a prerequisite for the S-specific NAD(P)H-hydrate dehydratase to allow the repair of both epimers of NAD(P)HX. (6R)-NADHX = (6S)-NADHX (6R)-NADPHX = (6S)-NADPHX Binds 1 potassium ion per subunit. Belongs to the NnrE/AIBP family.