Originally Posted by MSPeconomist

For some of the flights I checked, my intuition (based on patterns of how full they were, the timing, and what fares were available) says that a few upgrade seats should have shown as available, although I didn't do the exercise of buying and trying to upgrade. Upgrade availability can't be checked on delta.dumb because you can't get a full Y fare to show as there are cheaper -UP unrestricted P fares being sold. (I suspect they're expensive M-UP or B-UP but I didn't check.)