Complex sentence: Before upgrading to NCAA Division I in 2006, UNC was a member of the Rocky Mountain Athletic Conference from 1923 to 1972 and the Great Plains Athletic Conference (1972--76).
Split into simpler sentences: Before upgrading to NCAA Division I in 2006, UNC was a member of the Rocky Mountain Athletic Conference from 1923 to 1972. Joining the Great Plains Athletic Conference for four years (1972--76).