In situ cathodoluminescence experiments have been performed to follow the time dependence of the UV luminescence in epitaxial lateral overgrowth GaN specimens. The decrease of the observed intensity and red-shift of the UV peak are interpreted in terms of non-radiative defect introduction and diffusion. This leads to strain relaxation of the GaN epilayer, which is initially under compressive strain. Monochromatic UV and yellow CL images show that dislocations act as efficient non-radiative recombination centres, that they are not at the origin of the yellow band and that they do not move under the electron beam.