Hillary is so full of dung
Hillary Clinton's accent evolution (1983–2015)
Interesting mix. Some barely perceptible shifts, but then living for decades in Arkansas, as the video notes, would render at least familiarity with another voicing. As far as adapting to the audience, we all do that. It's called "mirroring". You use one voice to your boss, another to your pet, another to your child, another to a prospective S/O, et cetera, and when we hear the other party respond we "mirror".
Ask black people about the difference in mirroring depending on whether they're talking to a black or white audience.
I shifted my accent for broadcast work. Back in radio school we all did -- we read a piece of copy at the beginning of the course and listened to how we'd progressed smoothing out to 'standard broadcast American English'. Once you do that it can and does become the permanent default --- until you get fatigued, when the original starts to creep back in.
But today, if I'm in town I'll slip in a local Appalachian expression here and there to converse. Why not? It serves communication. If I'm on the air, that disappears and we go back to Standard.