I also paid $+miles to upgrade a leg last year when I booked ORD-LHR, but knew that I would only be waitlisted. When the upgrade did not happen it took a couple months to get the money credited back to my Amex, though the miles were credited sooner. (and it did not happen automatically--I had to fill out the request for refund online) I was so annoyed that I would not do this again unless the upgrade was actually available at booking.