This paper describes a detailed analysis of the effects of high temperatures on the optical performance and structural characteristics of GaN-based LED structures with a high threading dislocation density. Results show that, as a consequence of storage at 900 °C in N2 atmosphere, the samples exhibit: (i) an increase in the efficiency of GaN and quantum-well luminescence, well correlated to an increase in carrier lifetime; (ii) a decrease in the parasitic luminescence peaks related to Mg acceptors, which is correlated to the reduction in the concentration of Mg in the p-type region, detected by Secondary Ion Mass Spectroscopy (SIMS); (iii) a diffusion of acceptor (Mg) atoms to the quantum well region; (iv) a reduction in the yield of Rutherford Backscattering Spectrometry (RBS)-channeling measurements, possibly due to a partial re-arrangement of the dislocations, which is supposed to be correlated to the increase in radiative efficiency (see (i)).