That's IT! I was unaware of t T-24 upgrade. I checked the exact time of the email notice and it was within 3 minutes of T-24.

Since the upgradeable flight was the second leg of the ITN, it was actually confirmed about 25 hours before flight time. For SDC only the first segment must be within the 24 hour window.

Thanks very much. This now makes complete sense.

I will have to look and see if the T-24 is a published phenomenon!