Originally Posted by
xliioper
I have had a similar experience after a schedule change (with a revenue ticket). Booked a trip to HND in May and initially the system would allow me to make changes using Modify Flight option and quoted fare differences online. However, after a schedule change impacted the itin, it now returns an error (#600104R) if I attempt to make changes and says it can't be changed online and must be done by an agent.
Be sure to use the DL web site and view the reservation. I recently had a (minor) schedule change on a ticket and was unable to change anything using the app until after I used the web site to "accept" the new itinerary. Once I clicked that "accept" button on the top of the itinerary display on the web site, then I was successful making changes.