Catalyzes the conversion of glucosamine-6-phosphate to glucosamine-1-phosphate (By similarity). Glucosamine-1-phosphate is used for cell wall biosynthesis (Probable). alpha-D-glucosamine 1-phosphate = D-glucosamine 6-phosphate Binds 1 Mg(2+) ion per subunit. Homodimer, may form a complex with CdaA. Constitutively expressed, part of the cdaA-cdaR-glmM-glmS operon (PubMed:23192352). Activated by phosphorylation. Belongs to the phosphohexose mutase family.