Ok so i am not a denier of the party switch, frankly i could care less (I’m libertarian) but something isn’t adding up. If the parties switched during the southern strategy then how do we explain the classical liberal, laissez-faire republicans before that? For example Hoover, and Coolidge. They were certainly not progressives or a modern liberal.
More can be said, but you may find helpful this old answer by /u/Samuel_Gompers that addresses the different wings of the republican party: Why was there such a huge shift in the core viewpoints of the Republican party in the past 100 years?
For more on the party switch in general, the FAQ has a whole section on it.