Complex sentence: The total number of districts was reduced from 58 to 55 but due due to considerable population shifts over the course of two decades, some ridings were merged to form one while others needed to be split in two while some were unchanged.
Split into simpler sentences: The total number of districts was reduced from 58 to 55. Due to considerable population shifts over the course of two decades, some ridings were merged to form one while others needed to be split in two while some were unchanged.