Surface nanostructures migrate as a consequence of atomic diffusion. Under the effect of a force, arising for instance from an electric current or a thermal gradient (electromigration or thermomigration phenomena), the atomic diffusion is preferential in specific directions and affects the nanostructures making them move and change shape. In this work, based on Kinetic Monte Carlo simulations, we show the impact of an external force on the shapes of 2D atomic islands and vacancy clusters located on homoepitaxial (111) surfaces. At different temperatures, we identify critical values of the strength of the external force applied to the edge atoms, that lead to a series of transitions of the morphology of both islands and vacancy clusters from hexagonal to triangular-like shape. The shape variation is strongly dependent on the external force direction and on the step edge anisotropy.