Soaking crystals of an Au^I₇₂Cd^II₄₀Na^I₄ cage-of-cage in aqueous Co(NO₃)₂ afforded an analogous Au^I₇₂Co^II₄₄ cage-of-cage, accompanied by the exchange of Na^I and Cd^II by Co^II with retention of the single crystallinity. The homogeneous progress of the transformation led to the direct observation of intermediate species by single-crystal X-ray crystallography.