What happened in Atlanta is the inevitable end result of "conservative" pro business policies. They attracted large corporations which attracted lots of left leaning transplants. If Cobb had the demographics of 1994, it would still be red.
I also think generational turnover is a significant factor. The grandchildren of a lot of the voters in 1994 have now entered the voting-eligible population and we're not voting 75% R like our grandparents were. It's pretty clear in Atlanta that voters younger than 45, whether they are native born or transplants don't vote anywhere near as R as voters 60+, particularly among the white population.