I've heard some stories about those who ordered through AT&T and cancelled their order after they found out some stocks available at their local apple store. They cancelled their AT&T order but didn't get the eligibility back right away and had to walk out empty handed. They even called AT&T and apparently they had to wait 3-4 days.
It's not worth cancelling your order. If you can wait, I would wait. Or just buy the device in full price.