I recently booked a round trip flight from CVG to BKK in nonrefundable Premium Select with the plan of using cash or skymiles to upgrade to Delta One on the longer segments (if the price was reasonable). At time of booking earlier today, probably 75% of the seats on the plane were available, including almost all of the Delta One seats. Immediately after booking, ALL of the seats in first class/Delta One for ALL segments now say: " This seat is currently unavailable and could be occupied by the time your flight departs."

I contacted the Delta text help line asking if an upgrade could be purchased. They said that the "aircraft is currently sold out." I find it hard to believe that an entire Airbus A350 for a trip >100 days away sold out in a matter of seconds. Am I missing something, or will these seats eventually become available for upgrade? Can you not upgrade a "nonrefundable" premium select ticket? I'm silver medallion if it matters.