The website is finnicky about making changes after schedule changes in these circumstances (Companion Certificate, Upgrade Certificates applied, etc.). You simply need to call Delta. Have preferred flights that you'd want to switch to ahead of time to feed to the agent. If you get any pushback from the agent, either HUCA or affirm and reaffirm that you are requesting this as an even exchange due to the schedule change on Delta's part.

ETA: you should be able to switch to any itinerary that day. You also do not necessarily need to connect in ATL (although that may be the preferred option still). There is an ATL-FLL flight that gets you into FLL close to your original time (I'm a bit surprised you weren't auto-rebooked to this but DL's IT systems aren't always known for being logical to us passengers although it could have also been something about it rebooking you to a new flight that had companion certificate availability even if this shouldn't have mattered in practicality).