Changing your flight on the day of departure is what we call Same-day flight change in Delta Airline. In the same day flight change policy, you can change your flight if there is any seat available in other flight on departure day. To make same-day flight change delta airline charge $75 for one way and in confirmed status. During the same day flight change, you get two options as confirmed and standby.
Delta Same-day confirmed: This is the best option to select in which you can call delta airlines helpline or check on the website and confirm if any seat available in other flights. If any seat is available on other flights during the same day you get the seat status confirmed and get charged of delta ticket change fee i.e. $75.
Delta Same-day Standby: In this option, if no seat is available on same day available flights then you might have to arrive airport early and wait for your staus to get confirmed in earlier flight. This option is much riskier as compared to the same-day confirmed status but you will not get charged until you get the seat confirmed in any other flight. This option is only allowed for refundable and non-refundable tickets but not for basic economy ticket. Moreover, this option is only available for United States domestic flights.
In delta same day change policy you may get your seat confirmed in other flight with the same amount as the original flight but with some conditions. You can only change the flight with the same class, there is no option to upgrade your seat. Same-day flight changes are not allowed for the flights from connecting airport to destination.