What will replace the 485-based "Joyful Trains"?

[Sacto1985]

I've been reading that railfans in Japan are increasingly anxious that JR East may be starting to rapidly phase out the 485 Series EMU-based Joyful Trains.

 

I wonder what will be the replacement, though. Will it be modified 651 Series EMU's, which already have dual-voltage capability (after all, they were originally built to run on the JR East Jōban Line between the Tokyo area and Sendai)? The Izu Craile train could be a preview of other 651-based Joyful Train conversions.