what a bummer bc their website tells me other wise(well leads me to believe you can do it free of charge) and no where on this page did it mention the word international
http://www.delta.com/content/www/en_...-changes.html#

Eligibility

You can use the same-day confirmed option for travel within the United States, Canada, Puerto Rico, and the U.S. Virgin Islands on Delta and Delta Connection® flights.

You may request a same-day flight change up to 24 hours prior to the departure time of your original flight; however changes are limited to flights departing on the same day of ticketed departure.

If you have a flexible fare (usually a refundable ticket), you may be able to change your itinerary without a fee.

If you have an Economy fare, you can only be rebooked in the same fare class as your original flight.

If you have a premium cabin fare, -- including Delta One™, First and Business Class -- you may make a same-day confirmed change as long as a seat is available in the premium cabin

^^^^^^^^
Award Tickets are same-day confirmed eligible.
^^^^^^^^

Origin, destination or co-terminal (LGA to JFK) changes are not same-day confirmed eligible.

Same-Day Confirmed changes from connection to non-stop flights are not permitted.

Basic Economy fare (E) for flights flown on or after February 1, 2015 are not eligible for same-day confirmed travel changes.