There is some truth in this article. Twenty years ago you scarcely ever met someone who was Catholic down here. Then when the Yankees started coming, that changed- that, and the Mexicans....
You are right... the numbers do not reflect “conversions “ but relocations..just as the number of liberal democrats is increasing.. My guess is that the catholics in the south are no more observant than the ones in the north ..
The change isn't just in the last 20 years but goes back decades to the mass migration of people from the Rust Belt to the Sun Belt. More recently the influx of Hispanics into the South has increased the number of Catholics.