As revealed in Death Metal and was further confirmed by Scott Snyder in interviews, characters will now remember their experiences from previous continuities.
Everything that was once part of the main universe at the time of publication is now remembered, even if elements from different continuities may contradict each other.
So Superman can now remember Golden Age, Pre-Crisis, Post-Crisis, New 52 and Rebirth.
Jon Kent knows the absolute continuity mess behind his origin and so on. Everyone is now Hawkman.

But... How will all this actually work?
What rules should be applied so DC doesn't become even more of a continuity mess with this change?
Let's discuss