... I guarantee it won't be in the old universe.
The way I see it, TPTB will likely be deliberately unclear as to whether it's the old prime, or the new alternate universe. To the majority of the "non-hardcore" viewers it really won't make the slightest difference.
Unless they use ship designs, specific events, of have guest appearances from actors from the movies, the show could just as easily be in a third universe, or in the prime.