The leader of the UNU was Oleg Goltvyansky, until July 15, 2012 when he was replaced by incumbent Vitaly Krivosheev.
Since July 15, 2012 the head of organisation became Vitaly Krivosheev.
Since July 15, 2012 the head of organisation became Vitaly Krivosheev.
Make the sentence simpler: The leader of the UNU was Oleg Goltvyansky, until July 15, 2012 when he was replaced by incumbent Vitaly Krivosheev. Output: