I tried for an hour and a half on apple site no good. I went to AT&T's site and successfully placed an order on the first go, they whacked me an $18 upgrades fee, but I got the order thru for a white 64gb iPhone, try there if you are having no luck
I've been trying on AT&T since 2:01 CST and have only been able to get past check upgrade status once...and it timed out during checkout. Since then...nada.
AT&T is screwing up Apple's site as well. This is a joke.