Yellow Luminescence (YL) band and blue luminescence (BL) band in a studied unintentionally doped GaN sample show a transient behaviour where the observed luminescence intensities change with the exposure time of the sample under 325 nm laser beam excitation at 10-300 K. Such an intensity variation is accompanied with a red-shift for YL peak at 10-140 K and one for BL peak at 140 K. We propose that such behaviours are related to the chemical transformations of YL-related CN and CNON defects, and BL-related CN-Hi and CNON-Hi defects during the exposure.