We'll see how this plays out because as it stands with the books as we know them, this doesn't make much sense. All of the books seem to be pointing at the end sign for all of our current incarnations of the characters. And things like Alfred being dead and Batman marrying Catwoman have been explicitly confirmed to be things that are only possible because 5G/Generation 5 will be a legit shift.
So I don't think the books will just go on with their current set ups while some sort of 5G line is created.
What I'm expecting is something like what DiDio said about Generation 2. That's the generation we're all most familiar with. That's when Superman and Batman were in their prime and the original Teen Titans were just starting (that means classic love triangles, identities, and set-ups). DiDio said pointed to that generation (along with Generation 1) and, playing coy, said "hmm, I don't think we've see stories from these two generations". He later expressed himself less ambiguously when he said that they would be straight up rewriting and reimaging classic stories from those eras.
It was rumored that Black Label would be apart of this somehow and maybe be the platform to tell these gen 1 and gen 2 stories. Then the rumor was to use the giant sized books. But the idea my just be to set aside some books in the main line to do the gen 1 and gen 2 stories while the forward momentum of the line remains with the 5G books.
I mean, what good is making a fairly conclusive timeline with past events that we've never seen before if the idea isn't to explore it with about as much thrust as the stories set in the new unwritten present? Seems like this was always the plan in some from, but the method has been going under reconstruction.
I genuinely believe that we are on the last months of the present day DCU, and once 5G hits it will split the line between Generation 5 and gens 1 and 2.
Edit: I bet you that's why all the Legacy numbering is coming back. They'll keep those going, and then start the 5G books with new #1s and a 5G banner. And Gen 1 or 2 banners as needed for the other books so you know what's what.