This paper investigates the application of differential evolution (DE) algorithm for the problem of optimal placement of phasor measurement units (PMUs) in an electric power network. The problem is to determine the minimum number of PMUs and their respective locations to make the system observable. In order to cope with the continuous changes in the power system's topology, the optimization problem is formulated considering not only the new PMUs to be installed but also the existing ones to be retained or relocated. Additionally, the new formulation takes into account the locations, if any, at which PMUs shall or shall not be placed. The effectiveness of the proposed method is verified via IEEE 14-bus, 30-bus, 39-bus and 57-bus standard systems.