Converts N-acetylmannosamine-6-phosphate (ManNAc-6-P) to N-acetylglucosamine-6-phosphate (GlcNAc-6-P). an N-acyl-D-glucosamine 6-phosphate = an N-acyl-D-mannosamine 6-phosphate Amino-sugar metabolism; N-acetylneuraminate degradation; D-fructose 6-phosphate from N-acetylneuraminate: step 3/5. Belongs to the NanE family.