In YESTERDAY'S ENTERPRISE, at the end it's suggested Guinan has, on some level, an awareness of what's happened, and out of the blue asks Geordi to tell her about Tasha Yar.
We saw at the start of the episode that the Enterprise D's bridge and its crew were altered by the arrival of Enterprise C (and the timeline thus being altered).
These two events in themselves are in step with earlier Trek, such as CITY ON THE EDGE OF FOREVER, which show that in the TREK universe, history CAN be changed, and a single timeline is thus altered.
A new separate timeline is NOT created.
If that were the case, then Kirk and the others in CITY never actually returned to their Enterprise, but rather an Enterprise in a newly created third timeline. I do not accept this idea, and you'd have expected Spock to have said something if it were the case.
No.
In the TREK universe, history can be changed and even restored, but new timelines are NOT created.
This means one of two things-
1) The original timeline HAS indeed been overwritted by the events in JJ Trek, or
2) JJ Trek is a parallel universe to the original, which has now had its past altered. Were Spock and Nero from the original Trek universe we know, or were they from the JJ Trek universe? I dunno.
I like to think it's the latter, so that not only does the original universe remain in place, the JJ-verse may even have "our" Spock in it (possibly unaware he's in the wrong universe, since differences could be due to the changes 25 years prior to his arrival).