So long as you keep same origin/destination and cabin, you can change any Delta ticket on Delta flights originally scheduled for July 1st-4th. No change fee and no fare difference. Also applies to Basic Economy. You can also change the days of travel (out to the 8th). Link below.
Delta really wants to get people moved off of flights over this busy weekend, as I'm sure they're expecting a tsunami of delays and cancellations.
https://www.delta.com/us/en/advisories/other-alerts/july4th-holiday-travel-waiver