actually one of the theories of the mirror universe was it was a divergent universe at some point past in history. and we saw the other parallel universes in tng parallel episode/ so no this isnt new.
THEORY is the keyword there...
Lets stop using maybe and ifs, It become clear every single plot using time travel from TOS to ENT and the movies inbetween that had dealt with time travel has shown on screen evidence that any alteration in the past affects our prime universe's future.
Now in TV terms we can easily say its a new Universe but using TREK rules and cannon, far as the characters are concerned there prime universe has been changed.
we have DVD's etc so I don't see the problem with fans getting to grips with the timelime changes.