Readily available 1-mesyl-1,2,3-triazoles are efficiently converted into a variety of imidazolones and thiazoles by Rh(II)-catalyzed denitrogenative reactions with isocyanates and isothiocyanates, respectively. The proposed triazole-diazoimine equilibrium results in the formation of highly reactive azavinyl metal-carbenes, which react with heterocumulenes causing an apparent swap of 1,2,3-triazole core for another heterocycle.