Aragonite speleothems are increasingly being used as high-resolution climate proxies; however, aragonite is unstable and susceptible to diagenetic transformation into calcite leading to mineralogical, textural and geochemical alterations that have not been fully investigated. To provide some insights to these modifications, this study combines stable isotope geochemistry and U-series dating with petrological observations and EMPA elemental analyses to characterize a stalagmite from Castañar Cave (Cáceres, Spain) that shows primary aragonite diagenetically altered and with reverse ages. The diagenetic processes include recrystallization to calcite, micritization and dissolution. Characterization and interpretation of the chemistry and nature of the waters from which the aragonite precipitated, and of the ones that subsequently formed the calcite, indicates that both polymorphs formed from the same fluids. These fluids hydrochemically evolved from an “aragonite mode” to a “calcite mode” along a diagenetic path within the sample. The results show that during transformation the system stayed close in one of the stages, and open in the other with the same textural result. Reverse ages are found either in the recrystallized areas or in the primary aragonite, caused by two different U-remobilization mechanisms, U-leaching due to flowing waters on the surface, and U-loss during the transformation process. These results highlight the complex behavior of speleothems during diagenesis and the impact on the geochemical information of the primary and secondary phases, and thus the importance of petrological and geochemical characterization of speleothems for paleoclimatic studies.