Interaction lowers the energy of the occupied orbital, which lowers the energy of the system.
At the same time the unoccupied orbital is destabilized, which has no effect on the overall electron energy. There should be sufficient spatial overlap between the orbitals involved, the more the better. If the interaction is small, the 'new' orbitals will closely resemble the original mo's. Interaction is higher if the energy difference between the HOMO and LUMO is smaller . The new orbitals will become more and more mixtures of the original ones.