Make it clearly an AU but still not too different from the original timeline/universe, so no old Spock or any mention of the events in the original timeline/universe. It would be better if there was no question of it being an an altered timeline instead of an AU. AUs are canonical in Trek, after all. The plot wouldn't have to change a lot.
They say in the film that it's an alternate universe steming from Nero's incursion into the past in 2233. Before that it was identical to the ENT/TOS/TNG/etc timeline. Where's the confusion?