My lecturer says that molecular orbitals don't change when electrons are added or removed from a molecule, but I don't understand why. As electrons are described by wave functions, surely more or fewer electrons means that the electrons interact differently and consequently the atomic orbital wave functions interact i.e. interfere differently to form different MOs.
The molecular orbitals do change when electrons are removed or added. But what does not change is the relative order of these orbitals with respect to other orbitals. If a given orbital has a higher energy than another one, it will remain so if you add one electron or it you remove one electron, whatever the absolute values of these energies.