This still seems to be an issue. I'm trying to change my booked flight from ONT-ATL-JFK to ONT-ATL-LGA. I booked in F with miles.
When searching for a change it shows as available for a 0-mile difference, but when completing the transaction it errors with "The fare for the flights you selected just sold out. Please search again to view and select alternate flights. ROR4259".
I've been trying to make this change since yesterday, and it continues to show as available in new searches online and in the app. I messaged and Global Ticketing Support told me to cancel and rebook online, but the same price is not available for new bookings.