I am looking at the Delta web site right now, and I see that for same-day STANDBY, that the new flight must be earlier than the original flight, but no such language for same-day CONFIRMED.
I am, however, seeing a second devaluation: purchased Comfort now has the same rules as purchased main cabin (same fare bucket availability vs. anything in the cabin for sale, as for F / D1 / PS).


https://www.delta.com/us/en/change-c...-flight-change
SAME-DAY CONFIRMED

If you purchased a seat in Delta Main or Delta Comfort, you can switch to a confirmed seat on a new flight based on availability in your originally purchased fare class.  Delta Main Basic (Basic Economy) tickets are excluded.

If you purchased a seat in Delta First, Delta Premium Select or Delta OneŽ, you can switch to a confirmed seat on a new flight based on cabin availability. 

• Same-Day Confirmed fee is $75 USD
• Same-Day Confirmed is complimentary for Diamond, Platinum and Gold Medallion Members and for Extra and refundable tickets (See Terms and Conditions)

SAME-DAY STANDBY

You may wait for a seat on an earlier flight than your original ticket, if the Same-Day Confirmed option is not available.

• Same-Day Standby is a complimentary service at no cost to you
• Until you have been assigned a seat on an earlier flight, your original flight will remain confirmed