We report on the observation and manipulation of a two-dimensional crystal formed by a positively charged discotic polycyclic aromatic hydrocarbon at the liquid-solid interface. Using scanning tunneling microscopy (STM) as a tool, the supramolecular scaffolds of charged molecules could be switched between dissimilar polymorphs of different molecular densities. The observed phase transformation was found to be driven by electrical parameters such as magnitude of change of the substrate bias and voltage pulses applied to the STM tip. We conclude that the electrical manipulation of these charged molecules is a result of the creation of large local electric fields that interact with the adsorbed ionic molecules and thus cause molecular rearrangement.