Complex sentence: Not only was there a shift in population to the suburbs, there was also a shift in retail shopping as well; retail sales in the cities declined a percent between 1972 and 1987, having increased 63 percent in the suburbs.
Split into simpler sentences: Not only was there a shift in population to the suburbs, a shift in retail shopping occurred as well. Retail sales in the cities declined a percent between 1972 and 1987, while having increased 63 percent in the suburbs.