Involved in the biosynthesis of the galactose-containing O-side-chain polysaccharide backbone structure of D-galactan I which is a key component of lipopolysaccharide (LPS). Catalyzes the interconversion through a 2-keto intermediate of uridine diphosphogalactopyranose (UDP-GalP) into uridine diphosphogalactofuranose (UDP-GalF) which is the biosynthetic precursor of galactofuranosyl residues. UDP-alpha-D-galactose = UDP-alpha-D-galactofuranose Binds 1 FAD per subunit. Bacterial outer membrane biogenesis; LPS O-antigen biosynthesis. Homodimer. Abolishes lipopolysaccharide (LPS) biosynthesis. Belongs to the UDP-galactopyranose/dTDP-fucopyranose mutase family.