What would this look like though?
Many things in life change because of declines in health, fertility, appearance. Would these still happen? Happen slower? or you're "old" for 120 more years? If biology still works the same. Many of these things wouldn't change so dramatically. People would have to work harder to try and get retirement while they could. People only working for 1/4 of their lives would cause problems? Then again, those people could be more valuable? A healthy body might be more valuable than a healthy mind? No, not for most people anyway. Such changes might increase the development of automation.
If biology is tweaked, then things would be way way different. If static, people might live like their 20s and 30s? Relationships might become more tenuous? Granted one might not be able to isolate it from the rest of societal change, but life expectancy has changed. What can be attributed to it?
People might put off having kids until 150. One hopes young enough to see them grow up. More people might not have kids, A sudden change could cause some resource issues. Everything is currently set up for people to die (and stop consuming resources).
People might get more education? People can work for a few decades then get more degrees. People could go through (more) cycles of retirement / school / work?
… all kinds of things / too many ideas.
Reminds me a bit of 'In Time' (2011) https://www.imdb.com/title/tt1637688 mostly it's about other stuff, some of it interesting, but they do touch on some relevant ideas.