Actually it's only AA that doesn't recognize U because it's not a valid revenue fare bucket for them. DL U is their next to lowest economy bucket. Only T is lower. If the upgrade were to clear before the airport has control of the record AA's system would completely cancel the segment and the ticket. The situation would not be as dire on a DL issued ticket since their system would think there was a change made to the ticket but the new U class would probably not match the ticketed fare class. It would only cause the record to fall out of sync but the segment would still be on the PNR but in an unconfirmed status. If the flight was overbooked you'd lose your seat for sure. If not you'd need to have an AS agent fix your PNR at the airport and they'd probably have to take you out of U to do that. Once changed from U back to your original fare class that U seat would most likely be snapped up by the next person on the airport waitlist so in the end by getting on the waitlist early you'd be potentially causing yourself a lot of unnecessary grief and would probably lose the upgrade in the process anyway.