The first example of a reversible [Cu(4)] <--> [Cu(6)] interconversion for polynuclear copper(I) complexes under controlled experimental settings is reported. It illustrates the key role of specific crystal growth conditions for accessing the target cluster nuclearity that consequently determines physical properties of the resulting solid state products. Thus, when copper(I) benzoate crystallizes from a 1,2-dichlorobenzene solution at room temperature, it forms [Cu(4)]-core based crystalline material, [Cu(4)(O(2)CC(6)H(5))(4)] (1). In contrast, crystal growth by deposition from the gas phase at elevated temperatures results in the exclusive formation of [Cu(6)(O(2)CC(6)H(5))(6)] (2). Complexes 1 and 2 have been isolated in pure form, fully characterized, and reversibly interconverted into each other. The effect of a core structure on the spectroscopic properties of 1 and 2, such as IR, Raman, and photoluminescence, has been investigated. Additionally, a combination of X-ray powder and single crystal diffraction methods has been used to discover the temperature induced phase transition in the hexanuclear copper(I) system. Two modifications of 2 exhibiting slightly different solid state packing of the [Cu(6)(O(2)CC(6)H(5))(6)] units have been identified at room and low temperature. Moreover, reversible single-crystal-to-single-crystal transitions between these polymorphic forms have been confirmed. The important role of weak intermolecular interactions between polynuclear copper(I) units in the solid state has also been revealed and discussed.