Mentioning previous answers is intended to be a starting place, not to discourage your investigation. Further questions, data, and debate are welcome.

For the meta part: It's been asked before, but there's no need to delete the question. The rules do not discourage repeated questions. And if the rules do prohibit something, be assured that the moderators will, with remorse, remove it without any action on your part. Though do be warned: the way Reddit is coded, it'll still look like it's there to you, but only to you.

You can search using

• the search box at upper right (in the Web interface), which often works well enough but sometimes not

• https://redditsearch.io

• Google search on the Wiki specifically via, for example,

  "democrat" "south" inurl:reddit.com/r/AskHistorians/wiki

For the immediate question, there's an FAQ section on the US History page, https://www.reddit.com/r/AskHistorians/wiki/us_history#wiki_changing_role_of_republicans_and_democrats . One deals specifically in its title with the South, but that's part of the reason for the switcheroo overall.