June 29th will be just as much "too late" as July 9th will. I think it honestly depends on supply issues at this point. My guess is they'll want us to get the phones the same time they launch in stores, plus or minus a day at most. If they get absolutely slammed with preorders, I can see them waiting until the 9th to make sure they have enough to send us while still sending a decent shipment to each of their corporate stores. But if they correctly anticipated the huge demand for the phone, driven higher by the new upgrade policy, they should be able to fill the preorders and still ship some to the stores on the 29th. They want a unified launch, but if its more than a few days behind the other carriers I can see them losing quite a few potential new customers who don't care about the policies but just want a GS3 as soon as possible.
That said, this is Verizon and they are known for making decisions that are senseless and infuriating, as well as delaying everything from phones to OTA's for reasons that are rarely made clear. I can't imagine they haven't already completed extensive testing before announcing it, but one never knows.