Postsynthetic modification of metal-organic frameworks (MOFs) has proven an effective method of synthesizing architectures that have proven challenging to produce via classic solvothermal means. Herein we report the anion-cation assisted solid-state transformation of a three-dimensional MOF (ATF-1) to a series of two-dimensional structures (YCM-21-Z) via treatment with quaternary ammonium halides. It is important to note that this reaction requires no exogoneous building blocks (inorganic cation and/or organic linker) for conversion to occur. This reaction led to the synthesis of a chemically unique framework YCM-21-Bnpy in which phase-pure synthesis cannot be achieved by solvothermal means. The mechanism of transformation was studied with data supporting a nucleophilic halide (Br, Cl, or F) mediated de-intercalation and cation-assisted flattening of the In secondary building unit as the key mechanistic steps.