Involved in the synthesis of UDP-N-acetylgalactosamine (UDP-GalNAc). Catalyzes the conversion of glucosamine-6-phosphate (GlcN-6-P) to galactosamine-6-phosphate (GalN-6-P). D-glucosamine 6-phosphate = D-galactosamine 6-phosphate Belongs to the PGI/PMI family.