A facile method for post-synthetic exchange of anions in an iron(II) spin crossover material using mechanochemistry is described. Dry grinding of the [Fe(atrz)$$_3$$]Cl$$_2$$ complex (atrz = 4-amino-1,2,4-triazole) in the presence of an excess of sodium halide salt results in the complete exchange of anions and formation of [Fe(atrz)$$_3$$]Br$$_2$$ and [Fe(atrz)$$_3$$]I$$_2$$ in a solid-state metathesis reaction. The method represents a new strategy for tuning active switching properties such as the transition temperature in spin crossover systems. Formation of stable by-products was identified as a major driving force for exchange and a straightforward method to predict the likely outcome of such reactions using simple thermodynamic considerations is presented.