Did a confirmed SDC a week and some change ago on a confirmed First MGM ATL FLL itinerary. Was originally on the 6AM and wanted on the 9:30 out of MGM. App wouldn't let me do it and was making me pay fare difference which is in line with the website policy but the app SDC feature can be buggy sometimes. Decided to call in and was confirmed in First for no additional cost on the 9:30 flight.