Involved in biosynthesis of legionaminic acid (5,7-diamino-3,5,7,9-tetradeoxy-D-glycero-D-galacto-non-2-ulosonic acid)(Leg), a sialic acid-like derivative that is incorporated into flagellin via O-linkage to Ser/Thr. Catalyzes the conversion of GDP-N,N'-diacetylbacillosamine (Bac2Ac4Ac) into 2,4-diacetamido-2,4,6-trideoxymannose and GDP. It can also use UDP-N,N'-diacetylbacillosamine however it generates small quantities of 2,4-diacetamido-2,4,6-trideoxymannose. GDP-N,N'-diacetylbacillosamine + H2O = 2,4-diacetamido-2,4,6-trideoxy-alpha-D-mannopyranose + GDP + H(+) H2O + UDP-N,N'-diacetylbacillosamine = 2,4-diacetamido-2,4,6-trideoxy-alpha-D-mannopyranose + H(+) + UDP Belongs to the UDP-N-acetylglucosamine 2-epimerase family.