I think the plot device of someone waking up in their younger body sometime in the past but with all their memories of the future still intact has been done and redone to death.
However, I’ve never seen it done with this twist (or if it has I’d be curious read/see it). Say you wake up, or suddenly flash to, August 14th 2000 in the body you had then in the place you were then with all your memories up to August 14th 2020 intact. People not born before then are gone. People who died since then are back. But the twist is not only do you have all your memories of the future intact, so does every single person on earth.
How do you live your life going forward from there? How do others live theirs? What kind of ramifications arise from the situation? What macro-global-events does the world try to change or prevent? What kind of micro-personal-events do individuals try to change or prevent?
An interesting question, but the most obvious result is that the timelines diverge so quickly that not much would be the same. For example, I would be a young teenager. I guess I would contact my now-wife and we’d get back together? But I wouldn’t have a job…or would I? No, I don’t think the company I work for existed. But would it, if everybody remembered it?
Yikes, that goes all kinds of weird once you start thinking about it.
I would try not to get stabbed by my mistress in 2012. Hopefully she would try and avoid that as well.
I’d break it off with my then-girlfriend, before she screwed up my life.
Lots of FBI agents, knowing what they do now about 9-11, are going to do things differently in 2000.
This would be an awesome premise for a novel.
I have no idea.
Some kind of massive societal upheaval, right? You can’t force kids who have the experience and knowledge of 30-somethings to be kids. But what do you do about the really little ones? Can you give a 3-year-old with the memories of a 23-year-old legal adulthood?People who are in relationships that went bad are probably going to end them right away. I bet you have people who try to unite with their future partners and discover that the other person kinda wants to start over and do something different. Massive technological change and employment disruption as everyone tries to implement 20 years of improved process and knowledge.
A fun way to write this story would be that rather than everyone waking up at once, it happens over time. So you could have a small group of protagonists who find each other, then gradually realize that more and more people are becoming aware of the future.
Or she goes for a pre-emptive strike.
I agree wholeheartedly with walrus. Having awareness of the future gradually ‘expand’ helps build suspense and adds a whole new layer to the narrative. I’d read that book in a heartbeat.
I love the premise as well.
I think the OP would have to amend things, because that gets REALLY messy. Maybe have everyone over the age of 15 go back? You could have some event happen to everyone in 1985 that then has some sort of catalyst occur in 2000 (or 2020) that sends everyone back.
I also immediately thought of 9/11, and how we’d react. I’d think President Clinton would initiate some international coalition that goes directly after al Quida. But Bin Laden is also going to either go for a quick strike, or hide underground.
In addition to the global terrorism aspect, what about the economic scene? Amazon is already on the map, but they don’t necessarily dominate the logistics world.
One thing’s for certain in my life, though. I won’t be watching any fucking episodes of “The Apprentice”.
There’s also some “Minority Report” issues here. Do you go after people for crimes they haven’t yet committed?
That leaves you with weird boundary issues. Someone who’s a day older goes back but someone who’s a day younger doesn’t. Maybe you have people rewind 25% of their lifetime, which gets you the delayed effect with the oldest people experiencing it first, but doesn’t do really weird things with toddlers all of a sudden knowing the future. Anyone want to read a thrilling tale of octogenarians saving the world from 9/11?
I think Donald Trump’s life would be seriously at risk in 2000. Not to mention lots of serial criminals identified in the past twenty years.
Check out Ken Grimwood’s sf novel Reply for a very engaging, fun look at what one person would do if he went back to his younger self but retained his memories. Not the whole world doing it, but still.
You’ll have better luck if you look for Replay. 
That’s a fabulous idea - I too would read that book/see that movie.
One thing I wonder is if we would take our improved scientific knowledge (not just of science facts, but also of how circumstances play out under a known set of conditions) to do anything preemptive about things like global warming, pandemics, antibiotic resistance, and more.
Probably not, because humans suck. Now that I think about it, I guess in America we’d suddenly all be divided the way we are now. The die-hard Trump supporters would have 20 extra years to inflict their views on the US. 
I wake up one morning, and wonder why I’m waking up back in my old room. At first, I think I’m just visiting, but then I remember that my sister has taken over the room, and how did it get reconfigured the way it used to be? Slowly it dawn on me that I just had the most intense and realistic dream of my life. An entire lifetime and family simply dreamed into existence. What day is it?
I go downstairs and find my mother in the kitchen, sitting dazedly at the table.
“I just had the weirdest dream, I thought it was the year 2020” I say.
“So did I.”
“What’s going on?”
Panic sets in as we check with the rest of the family. Sirens blare in the distance. We turn on the television, and the world is in chaos. Then the nukes go off…
The End
When people start acting differently based on their “memories of the future”, then events will unfold differently, so after a short period of time these “memories of the future” will no longer be valid, they will no longer correspond to reality.
You can’t really get around the fact that this is in no way the same reality you are going back to, because the configuration of everyone’s brain is itself an integral part of reality. So this supposed “memory of the future” is really just knowledge of how things transpired in a similar parallel universe. That might give people insight into the possible consequences of their actions, and how to act more wisely, but it is not foreknowledge of the future in the universe they are now living in.
In principle, we can actually do this. It doesn’t require time travel. It requires the computational power to simulate enough of the universe that we can set up the conditions today and run the simulation forward 20 years (much faster than real time) with whatever variations we choose, and see how things turn out with slight differences in initial conditions.
Well, I’m gonna need to be retrained at the job I was doing twenty years ago. And I’m not sure who would be able to teach me how to work on a tool nobody has worked on in 20 years.
Also, my wife was 10 years older than me. So if this happened when I was 30. That would make her 20 and me 10. Awkward!
ETA: I should add, of course, that we may actually be living in one such simulation. There’s an argument that when the technology of a civilization advances to the point where it can simulate a universe, it will probably simulate a vast number universes, to see how they turn out. So there are likely to be far more simulated universes than “real” universes. Therefore our universe is probably a simulation.
Things could diverge on a large scale very quickly. Only a few people choosing to vote, or switching from 3rd party to major candidate changes the outcome of the 2000 election. Then it’s what would President Gore do to prevent 9/11, etc.
As a narrative device, it’s probably most interesting to go back to just before some major event, and consider how this “memory of the future” affects that single event (before any significant divergence occurs).
I think a good twist would be for everyone to have this “memory of the future”, but for people to have different memories, yet all be convinced that their own memory of the future is the correct one - and therefore have conflicting ideas on what to do.