Catalyzes the interconversion through a 2-keto intermediate of uridine diphosphogalactopyranose (UDP-GalP) into uridine diphosphogalactofuranose (UDP-GalF). UDP-alpha-D-galactose = UDP-alpha-D-galactofuranose Binds 1 FAD per subunit. Bacterial outer membrane biogenesis; LPS O-antigen biosynthesis. Homodimer. Belongs to the UDP-galactopyranose/dTDP-fucopyranose mutase family.