Delta's
domestic Contract of Carriage is poorly written, especially when it comes to schedule changes. Rule 240 provides:
C. Schedule Changes, Delays, & Flight Cancellations within Delta’s Control
When, as a result of factors within Delta’s control, you miss a connection due to flight delays, your flight is cancelled, or a substitution of equipment results in a change in the class of service that you purchased or prevents us from transporting you, Delta will provide you with the following:
1. Transportation to Your Destination
Delta will transport you to your destination on our next flight on which seats are available in the class of service you originally purchased. At our sole discretion, we may arrange for your travel on another carrier or via ground transportation. If acceptable to you, we will transport you in a lower class of service, in which case you may be entitled to a partial refund as set forth below. If space on the next available flight is available only in a higher class of service than you purchased, we will transport you on the flight, although we reserve the right to upgrade other passengers on the flight according to our upgrade priority policy to make space for you in the class of service you originally purchased.
2. Full or Partial Refund
If some or all of your ticket is unused, you may be entitled to a refund. Any refunds will be made as provided in Rule 260.
Rule 260(A)(1) (providing that an involuntary refund will be "an amount equal to the fare paid" if no portion of the ticket has been used).
Unlike that of other carriers such as NW, DL's domestic Contract of Carriage does not set a floor for when a schedule change is sufficiently onerous to vest you, the passenger, with remedies other than accepting the change and traveling on the new schedule. So, yes, as both a theoretical and practical matter, you should be able to seek a cancellation and a full refund, especially when you explain why this specific schedule change is not acceptable to you. Politely disengage and call back if you get an uncooperative agent.