At what point does the Prime Universe become the Kelvin universe? Is it judged on the how big a change to the Prime timeline is (the destruction of Vulcan being a big one), or is it from the point the Narada first appears and whoops poor USS Kelvin? My thinking is that with the Narada still flying around the galaxy doing nothing at the time Star Trek: Discovery takes place, you can still say the series takes place in the Prime Universe even though Nero is lurking around waiting to turn it into the Kelvin universe.
There are four possible answers that make sense to this question:
1. The point of quantum branching occurs when the Narada very first emerges from the wormhole. Everything that happens from that point forward is butterflied by that event from that point forward.
2. The Kelvinverse was *always* a parallel universe - the Narada and Spock Prime just shifted over (and back? maybe, but not necessarily if that universe was out of temporal sync with the Prime one to begin with).
3. Some previous temporal event, such as the events of "Star Trek: First Contact" or events shown on "Enterprise" as part of the Temporal Cold War, had *already* been the branching point for the Kelvinverse - and the Narada and Spock Prime only shifted over while going back through the wormhole.
4. (Proposed by Simon Pegg, if that matters to you, but still not screen canon) The point of quantum branching occurs when the Narada very first emerges from the wormhole, but, unlike answer #1, the changes propagate both forward AND BACKWARD in time from that event. So how far back before that point the changes would have occurred is hard to say and depends on how big a "ripple in spacetime" the sudden addition of the Narada created.
I'm a proponent of answer #3, myself, placing the point of divergence at the appearance of the Borg sphere and the Enterprise-E around 2063 Earth.
But I have to admit to finding Simon Pegg's idea interesting - especially the implication that if the appearance of the Narada made a finite ripple backward and forward, then at some point in the future, the Kelvinverse and the Primeverse should achieve mathematical identity once more - and whether that would result in identical but parallel universes or the actual reunion of both into one universe moving forward from that point but with 2 histories for a portion of them, I don't know.