I made the mistake of preordering Verizon but had second thoughts about losing my AT&T unlimited data, so I ended up with an October 21-28 delivery window when I ordered an AT&T upgrade from Apple.com
Contemplating trying my luck in store tomorrow, I called Apple and they said it would take 24 hours for them to process the cancellation and reset the upgrade eligibility through AT&T. Has anyone tried it themselves? How long did it take for you to become eligible for an upgrade again?