I watched a program a while back that I had found on a UK torrent site. It was about captured British soldiers during WWI. German doctors had recorded their voices in what was a study on culture and regional language differences. The producers of this program decided to try to locate relatives of some of these men and compare the language from then and now. Most relatives were still living in the area their WWI ancestor had been from. They played the recordings for the relatives, and asked them if they recognized the pronunciation of some of the words and phrases. In most cases, the pronunciation and use of those same words or phrases differed from the way the present-day family members said them. It was pretty interesting to see how much the language in their region had changed since WWI.
The actors Timothy West and Prunella Scales (Mrs. Fawlty) were so appalled by young British actors not understanding how English was spoken in the 1930s (something you should know if you’re doing Noel Coward!), they created a recording facility where young actors can go and hear voices from the 1920s, 30s, etc. Perhaps even earlier so actors can do Shaw properly (although I realize he’s a dirty word here!) Sounds a little like what you are talking about.
If you really listen you can trace the change of accent from the Center of Boston or as a friend of mine used to say Massachusetts ends at Worcester.