AbstractAll‐inorganic lead‐free Cs₂AgBiBr₆ double perovskites have gained significant attention due to their potential as stable and nontoxic photoactive semiconductors. Currently, it remains challenging to synthesize homogeneous microcrystals (MCs) exhibiting excellent properties, which are necessary for large‐area device integration. This work proposes a two‐step synthesis approach involving the introduction of a foreign silver cation to transform the 0D layered Cs₃Bi₂Br₉ to 3D Cs₂AgBiBr₆ perovskite structure. This work has studied the cation exchange (CE) transformation mechanism by isolating intermediates to be able to follow the evolution of the crystal lattice as well as the structural and optical properties over time. Moreover, complete CE results in phase‐pure, highly crystalline Cs₂AgBiBr₆ MCs exhibiting excellent photonic properties, superior to their counterparts synthesized by anti‐solvent precipitation. These findings highlight the potential of CE‐induced transformation as a means of synthesizing novel, stable perovskites.