Interesting how in "micro" sense we know people knew it was happening ("young people today.." nothing changes) Yet, its a 300 year process. One of those "I was there" -no, you weren't things. Like sea level rise. You see a bit of it. The totality is a story which spans generations. Nobody owns all of it.
And yet, people experience changes in accents during their own life; they hear recordings from themselves from the 70's and think "did I really talk like that?". One thing though, take older movies with a grain of salt; in old movies, actors were taught a specific accent (called the Mid-Atlantic accent, https://en.wikipedia.org/wiki/Mid-Atlantic_accent ).
Modern example. Router. Two forms, the British one rooter is being deprecated strongly for rowter.
Etc is etcetera. Now, its etsy.