I have always prepaid the early upgrade fee also, but I'm wondering if this is really necessary? Apple and AT&T have slowly but steadily improved their systems for handling these upgrade purchases. I'm wondering if I just preorder tonight, and select "upgrade on AT&T Next", wouldn't Apple's server get the upgrade costs from AT&T for my specific account and just tally up the bill accordingly? In other words, would not prepaying through AT&T at all slow down the preorder process?
I may take my chances that this is the case. I have an X already so I'm not as concerned with the potential that I'll miss the window and not get the Xs on launch day. Basically I'm upgrading because I'm already on an annual Next program, so why not upgrade to the better camera, CPU, and faster FaceID.