A new luminescent Cu(I) tetrametallic metallacycle B is reported, featuring very rare semi‐bridging aqua ligands. As heated markedly above room temperature, this compound undergoes in the solid‐state a post‐synthetic transformation, affording the new luminescent metallacycle C. Thermogravimetric analysis, IR spectroscopy and single crystals X‐ray diffraction reveal that this alteration preserves the gross tetrametallic macrocycle structure, however caused by the release of the coordinated water molecules with the concomitant formation of cuprophilic interactions. This transition induces a shift from eye‐perceived green (B) to blue (C) room temperature luminescence for these molecular solids. Photophysical measurements and time‐dependent density‐functional theory calculations have been conducted to identify the origins of the emission properties lying in these structurally related assemblies, suggesting that Thermally Actived Delayed Fluorescence dominates the radiative relaxation pathways. This study highlights the innovative feature of Cu(I) derivatives, offering the access to stimuli‐sensitive materials that can witness a posteriori the exceeding of critical temperatures in their environment.