The alignment of nematic liquid crystals in a hybrid cell can be easily reconfigured by applying an external electric field. We exploit this feature to realize a waveguiding structure whose refractive index profile can be dynamically reshaped. The mode structure of the guided light, investigated by m-lines spectroscopy at various electric field strenghts, can be well predicted by a parameter-free model based on the elastic, dielectric, and optical properties of the liquid crystal. According to such a model, the field profiles of the modes supported by hybrid nematic waveguides display very peculiar features.