SPG and Delta launched Crossover Rewards back in February (you can read more here). I broke the story few days earlier and had the original intended terms and conditions that did include space available upgrades on Delta flights for SPG Platinum elites (read more here).
This program feature was withdrawn before the official launch, however.
Now, Delta has updated the terms and conditions of the Crossover Rewards program and the day of departure upgrades for SPG Platinum members are part of the program. You can access the Delta’s web page for this program here, frequently asked questions here, and the terms and conditions here.