Originally Posted by
raistlin
Why? It's pretty common practice.
You need to go to the airport and ask either at check in or directly on board (yup, they'll get your payment with credit card).
BTW if you have AZ miles you can upgrade at the airport to any seat left open.
Common practice? Let's not exaggerate, I've been upgraded several times while boarding (found out when scanning the BP, although the actual upgrade occurred earlier), but after boarding? How will the FAs know the price, and when will they do it, while people are boarding or after take off? And, obviously, I am fully aware that upgrade at the airport is common practice.