The modification of high-mannose (Man9-Asn) N-glycans by mannosyl-oligosaccharide 1,2-alpha-mannosidase. This may result in Man8GlcNAc2-Asn N-glycans (which in yeast may be subsequently modified by the addition of further mannose residues) or Man5GlcNAc2-Asn N-glycans that are substrates for further diversification in the Golgi apparatus.