All's well that ends well - happy to report a very helpful response from FB on this one. Fortunately my original ticket had not been issued, so the original booking was simply cancelled. I rebooked, and the girl on the phone booked the ticket after checking that upgrades were available both ways. Within an hour, I had email confs of both the booking and the upgrade, plus confirmation of the mileage debit from my account. It means paying £300 out of my own pocket for the luxury of Delta BusinessElite, but it was worth it.