This will be interesting....
1) For OnStar users within major metro areas where AT&T has good (subject to debate) coverage the only issue I wonder about are any of the existing vehicles capable of switching over to AT&T? GM left a bunch of 04 and older vehicle owners with non-functioning OnStar due to a change in the past. Did they have the foresight to make current systems GSM capable?
2) For OnStar users outside major metro areas where AT&T has most likely crap coverage. These are the users that may actually need emergency services. They may be in an area with little or no traffic for assistance from passing motorist, nearly out of gas and trying to find a close (and open) gas station or other scenarios. What happens when these folks not have no coverage when they did before under Verizon.
It sounds like only 2015 and newer cars will use AT&T. Older cars will continue to use Verizon.
The 'change in the past' was the carrier's elimination of analog service, which many older cars used.