A confusing data point. On a transcon flight I had set up a waitlisted RUC for D1. Two other flights showed GUC availability online.I called to switch to one of those flights. I'm guessing that because of the waitlisted RUC I had to be transferred to a desk for a manual reissue. After a 90 minute hold I was by the manual desk that GUC was available on all flights- not just those with Z availability. So I was able to use the GUC on my original flight. The original representative had assured me a GUC was not available on my original flight.