the newer fw versions V400C02 and higher has a different structure in the settings. If you just downgrade by flashing some lower version "upgrade file" via GUI, the older fw will choke on the new version settings and your box may not respond any longer (need to do next step via recovery loader; sometimes it suffices to flash an old settings file (form fw version ~4b07) via recovery loader).

The best way to downgrade is to flash a very old fw (~V4b07). Even this fw needs to be rebooted twice to clean up all settings and work stable. 

After this you can again flash a different GUI in case of pearl boxes and then load a recent (not C Series) upgrade file via GUI and enjoy your box with new features.