Nanomaterials have exceptional properties often related to the large number and arrangement of their surface atoms. Depending on the crystal facets, activity and selectivity for specific reactions can be tailored. Accordingly, it is important to understand the structure of different nanomorphologies with their dominant facets varying and the transformation from one to the other. However, the mechanisms behind these transformations are typically not well understood, preventing the tailored synthesis and application of different nanomorphologies. Such understanding is further hindered in materials with highly complex structural features including multiple chemical compositions, stoichiometries and polymorphic crystal structures. In this work, we focus on the morphological transformation of 2D manganese oxides (δ-MnO_x) nanosheets upon galvanic exchange reaction with ferrous ion, a procedure often conducted to enhance their conductivity by Fe³⁺ doping leading to Fe_xMn_1-xO₂. An enhanced conductivity is often required for energy storage and electrocatalysis application. The morphology can be tuned from nanosheets to nanowires or the barely described nanocones by controlling the pH value or ionic strength during the galvanic exchange reaction. In order to understand the mechanism behind these morphology transformations, the structural features of these materials were investigated by high resolution (scanning) transmission electron microscopy (HR(S)TEM), electron diffraction (ED) and Raman spectroscopy. Energy dispersive X-ray (EDS) was used to determine the different local chemistry and iron contents in the nanomorphologies, while electron energy loss spectroscopy (EELS) revealed their local Mn oxidation states. Our combined characterization approach allowed us to propose and validate an oxidative mechanism of morphology transformation by the in-situ formation of Mn²⁺. This study highlights the importance of electron microscopy and spectroscopic analyses for unravelling mechanistic paths relevant for the design and growth of high-performance functional nanomaterials, directly contributing to build their structure-property relationships.