Just for some perspective-
DL has sold upgrades for years, but ONLY after all elites (and their eligible companions) have been upgraded. It's $150 each way for non-elites to be upgraded on a transcon, for example (but only from K or above fare category). Of course, DL is not as elite-heavy as CO... it's relatively frequent that ALL elites are upgraded on the JFK-West Coast transcons, for example, and then they are selling upgrades.
Whenever you are on DL flight and the GIS says "standby upgrade available", that means that all elites have been upgraded.