My dad is from a small town in eastern NC, right by the coast, where his side of the family had lived for generations. He moved to Charlotte after graduating from law school and his been there for over 52 years now. He still has a very noticeable southern accent, but when we would go back to his hometown to visit family, he would turn it up to 11. It would become much more pronounced than his everyday accent at home. (I haven’t been back with him to his hometown in over 30 years now, so not sure if that’s changed.)
My dad’s sister moved to Boston after graduating from UNC in 1973. She lived there for 20+ years, and then moved to California where she has lived for about the past 30 years. For as long as I’ve known her she has never had anything close to a discernible southern accent. When she was young she made an effort to get rid of it, as she wanted to get out of the south and feared that having a southern accent outside of the south would hold her back.
And then you have my mom’s side of the family. My mom is from upstate/western NY, where her side of the family had lived for generations. Pretty much all of my family members who still live there have that upstate/western NY accent, which is somewhat comparable to a midwestern accent. Interestingly, though, neither my mom, her brother, nor my grandparents have/had that accent. But so many of my cousins do, including cousins in my mom’s generation and cousins in my generation. Yet the accent was much less pronounced, and in some cases non-existent, in most people I knew from there in my grandparents’ generation and older. My theory there is that the accent was much more prevalent and pronounced among the working class (and working class folks up there to this day generally have a VERY pronounced upstate/western NY accent), and much less so among the middle and upper class folks. Then over time, the upper class in that area sort of died off/got old, and what was left of it, along with the middle class, mingled more and more with the working class. From there, the heavy accent that was prevalent among the working class spread to everyone else.