country singer stage dialect change Re: "Y'all" in Friday's New York Times

[Dan Goodman]

Beverly Flanigan wrote:

> And this reminds me of another puzzling comment from NPR last week:  The
> interviewer of a country singer (can't recall her name, but she didn't
> sound authentic to me; I've known people here who think they "talk
> Appalachian," for example) said that if listeners wanted to hear her "sing
> in Middle English" they could go to npr.org for that day.  I haven't looked
> it up, but I wonder if he meant
> mountain/Appalachian/Scots-flavored/hillbilly English?  Kinda like
> Elizabethan, maybe?  These historical eras are hard to keep straight, you know.

Something I keep meaning to ask about country music:

In recordings from the 1920s, almost  all country singers sound
Southern/Lower South to my ear.  (Exception -- one singer with a
definite London accent singing an English music-hall song:  "My Old
Dutch."  The music company representative who recorded him might not
have noticed the difference from the way the singer's neighbors
talked/sang.  Or, more likely, didn't care.)  This continues up through
the 1940s.

And then, in the 1960s and later, the singers sound South Midlands/Upper
South.   Or rather, a stage version of that dialect.  (I suspect that
performers who have that as their native dialect have to learn to use
the country music version.)

If I'm correct about this -- what happened to bring about this change?

If I'm not correct -- what/how have I misheard?