Board Thread:Questions and Answers/@comment-30536711-20160614204406/@comment-3361105-20160614210842

Prime Earth - or for convenience's sake, the Post-Flashpoint universe - is a timeline shift, not a different universe.

So, it's a bit complicated.

Functionally, what Flashpoint did was reboot the timeline from the beginning - which means that the New Earth versions of the characters would be erased by the timeline shift. So, some of pre-flashpoint history still applied for the post-flashpoint versions of the characters, but they were different versions of the characters, in different times in their careers. That fact required that we split their pages between New Earth and Prime Earth (both of which are basically fan-names for Earth-0).

So, until Convergence, there was no way for Wally West to exist unless he was written into the universe as a new character (which he was).

What happened in Convergence, though, was that Brainiac - who importantly lives outside of the DC Multiverse and can travel through time - went to different eras and universes and stole individual cities from universes/realities that were about to be destroyed or erased. (And New Earth was one of them).

This meant that pre-flashpoint characters who were in the stolen cities could co-exist with post-flashpoint continuity characters (as well as several pre-crisis multiversal characters and elseworlds. etc.)

At the end of Convergence, Brainiac basically allows the heroes to undo/prevent whatever crises would have caused them to be erased/destroyed, and they all live happily ever after in coexisting, super-messy continuity.

What appears to be happening in Rebirth (mind you, I've only read the DC Rebirth #1 so far), is that defeating Darkseid (who, according to Multiversity is a character who exists in all universes), broke something that let Wally West from New Earth escape from the Speed Force onto Prime Earth, and "remind" people of the past reality the was erased. And I have to stress that again: New Earth was erased. Prime Earth and New Earth are not the same thing.

So, Wally making Barry remember New Earth is a wonky thing, because it doesn't make sense, but this is comic books, so it can happen anyway. There's precedent for it in books that were written relatively soon after Crisis on Infinite Earths, but overall, the idea is that only special characters who have been lost in some kind of comic book limbo ever really remember the way things were.