According to Star Trek XI's theory, the Borg travelling back in time from 2373 to 2063 wouldn't have made a blind bit of difference to the existing timeline, since it would only have created a divergent timeline, leaving the original intact....so then how come the Ent-E crew saw Earth get transformed into Borg Earth in the 24th century, because that shouldn't have been possible. Earth should have stayed as it was. Maybe the temporal corridor thingie they were caught in somehow enabled them to briefly glimpse another reality. Whatever, the events of First Contact create three divergent realities: The 'original' timeline of Picard & co.; the Borg-assimilated timeline briefly seen; and the timeline where the Borg nearly assimilate Earth but are stopped by Picard and co.
To be honest, it's a lot easier for me to say that the new timeline as presented in Star Trek XI is, for better or worse, THE new timeline of the Trek universe, and the old one no longer exists. Of course, this would mean that any adventures that are set after the movie's point of divergence now no longer happen at all. Alternatively, in my opinion, the only way you could have this new timeline and the old version coexisting in the same multiverse is if the universe that Nero and Spock find themselves in after they leave their 24th century is already a separate reality from theirs before Nero blows up the USS Kelvin; in other words, when their ships got sucked into the singularity, they not only went into the past, but into the past of another reality entirely, one which shares a great many similarities with their own.