Changes of the toroidal mode number (n) of the edge localized modes (ELMs) have been observed during the ELM evolution phase in several KSTAR discharges. The mode transition always accompanined a discrete change of the mode frequency and it typically lasted for a few hundreds of micro seconds. While the global parameters (stored energy, plasma current, shape, etc.) did not change very much (~2%), the toroidal velocity gradient and the ion temperature gradient at the pedestal considerably changed through the mode transition.Two classes of mode number transitions were identified: overlapping transitions and non-overlapping transitions. In the former case, another filamentary mode structure appears and co-exists with the original mode structure and it persists after the original one perishes. On the other hand, in the latter case, the coherent filamentary structure disappears temporarily and re-appears with a different mode number during the transition. These phenomena may help to understand the previous observation on the ELM crash dynamics where a rapid mode number change preceded the ELM crash [1]. *Work supported by the NRF of Korea under contract No. 2014M1A7A1A03029881 and the BK21+ program.