My sister is a WP Elite and asked a Delta GA in Atlanta yesterday whether she could get upgraded on a DL flight. She was told this was possible but NW had to put her on the upgrade list within 24 hours of the flight.

I believe this GA gave sis some bogus information. I've never been able to score a DL upgrade as a NW platinum and am pretty sure there's no agreement for upgrades between Delta and NW.

Is there some hidden rule I don't know about?

Thanks!