The first time one pot reversible and quantitative interconversion of three crystalline polymorphs of a compound by ball mill grinding to milling equilibrium is here presented. A contaminant in a concentration as low as 1.5% mol mol -1 is required to enable all these three polymorph transformations. We propose the contaminant acts as a template for the polymorph conversion. The relative stabilities of the polymorphs must depend on the size of the nanocrystallites produced under different conditions and by surface interactions with small amounts of added solvent.