It is possible to create a "hacked" firmware used to install older versions.
Actually, there are two ways:
1. Prepare a firmware which "cheats" that it is a newer version, but contains older version inside.
2. Prepare a firmware which uses rollback functionality
(note that I'm not saying it's easy, only that it's possible - the firmware has multiple checksums which have to be recomputed correctly after any modifications)
DJI have a special file identified as firmware "99.99.9999" and it has a special "rollback" function enabled. If you ever looked as your update log, it is likely you've seen update to this version at top of log file. (I'm not 100% sure about what that special firmware actually does)
My sources for the above:
- looking at various update logs found on the internet
- reverse engineering camera firmware, ie. function DjiUegPktHeaderCheck()
EDIT: Also, reading from the first page of this thread - there seem to be other downgrade methods.