This stuff about politicians switching parties is a red herring anyway. Why would you particularly want your former political enemies to come over and join you if you can just steal their voters and replace them with politicians from your own party who you actually trust and who largely follow the same policy positions as you?
The Republicans didn't become the old Democratic party, they largely kept their economic positions but just won over Southern working class voters via the racist messaging and positions. Which is why you now have this quite bizarre dichotomy between a party of big business and the rich being voting for by large numbers of very poor people.
The flip-flop of parties and their platforms began in 1912 when Teddy Roosevelt pulled the Progressives out of the Republican Party to form the Progressive Party.
From its inception, the Republican Party had been an odd partnership of northern Big Industry and progressive moralists. Their initial goals were parallel, but for different purposes. Both wanted slavery abolished--one group for moral reasons, the other group for economic reasons.
After they achieved that goal with the Civil War, the Progressives of the GOP immediately turned the rest of their progressive agenda: Temperance, prison reform, racial and immigrant reconciliation, women's rights, environmental conservation, and labor reform. Teddy Roosevelt--who had been aggressively progressive from the very beginning of his political career--was their foremost evangelist who found the presidency an excellent pulpit.
But the moral Progressives of the GOP parted company with Big Industry in the GOP over environmental conservation and labor reform. Teddy Roosevelt pulled the Progressives out in 1912, leaving the Republican Party filled with libertarian-minded businessmen who were unable to win a national election. And the tiny new Progressive Party couldn't win an election either. That split allowed the conservative Democratic Party to begin winning.
Over the next few decades, Progressives in the north began registering Democrat as Progressive economic goals became clearly more anti-business. Progressives in the south tended to remain Party of Lincoln, where they were dealing with a different sort of antagonism to their social goals.
But when it comes to a "Southern Strategy," that was not invented by Nixon, but by Barry Goldwater. Goldwater opposed the 1964 Civil Rights Act, but on Libertarian grounds, not segregationist grounds. In fact, Goldwater was not personally segregationist. He had integrated his own family business, and he had been a staunch supporter and ally of integration of the federal government, particularly in the DoD. But he believed the decision to integrate or remain segregated should be made by individual business owners, not government at any level (and he was morally opposed to southern state and local laws that mandated segregation).
Yet...Goldwater wanted to win, and just like moralists and Big Industry became allies against slavery in the Civil War, Goldwater thought that a Republican alliance with segregationists on the Civil Rights issue might help him win the presidency.
There were some things that slowed the process. One was the rise to the national political level of southern favorite son George Wallace. It was Wallace alone who kept Nixon from sweeping the south in 1968, although Nixon did win all southern states (well, every state except Massachusetts, as well as DC) in 1972 . Southern Favorite son Carter also slowed the transition.
And another thing slowed the transition: Diehard Dixiecrats (voters and politicians) who consistently voted and legislated segregationist, but would rather die than actually register into the Party of Lincoln. Eventually by the 1990s, those diehard Dixiecrats...finally died. Their children registered Republican.
