Catalyzes two distinct but analogous reactions: the reversible epimerization of UDP-glucose to UDP-galactose and the reversible epimerization of UDP-N-acetylglucosamine to UDP-N-acetylgalactosamine. The reaction with UDP-Gal plays a critical role in the Leloir pathway of galactose catabolism in which galactose is converted to the glycolytic intermediate glucose 6-phosphate. It contributes to the catabolism of dietary galactose and enables the endogenous biosynthesis of both UDP-Gal and UDP-GalNAc when exogenous sources are limited. Both UDP-sugar interconversions are important in the synthesis of glycoproteins and glycolipids. UDP-alpha-D-glucose = UDP-alpha-D-galactose UDP-N-acetyl-alpha-D-glucosamine = UDP-N-acetyl-alpha-D-galactosamine Carbohydrate metabolism; galactose metabolism. Homodimer. Belongs to the NAD(P)-dependent epimerase/dehydratase family.