With the release of issue #8, there will be "doors open" that will definitely undo certain things done YEARS AGO... And that's pretty much a SPOILER in itself that would be telling.
Still here? Read on after the jump then. SPOILER WARNING AGAIN.
The DC Multiverse OFFICIALLY RETURNS, and what was undone and erased by 1985's Crisis of Infinite Earths is now undone. Everything exists in continuity, and all the past Earths and timelines explored can be revisited by writers.
You can get the full gist of things by visiting Newsarama's piece on it.
Basically... WOW DC FANS. Every kind of reality can be explored again!