UDP-beta-L-arabinopyranose = UDP-alpha-D-xylose Nucleotide-sugar biosynthesis; UDP-L-arabinose biosynthesis; UDP-L-arabinose from UDP-alpha-D-xylose: step 1/1. Cell wall biogenesis; cell wall polysaccharide biosynthesis. Belongs to the NAD(P)-dependent epimerase/dehydratase family.