I think, the main question should be then what this client did to do a downgrade. A revert from a snapshot should work to do a "downgrade" back to the state the device was.
Is it a management server or a gateway?
If it is a management, then the option would be to do a clean install of R77.30 and import old (R77.30) migrate export or a backup, and import of Gaia settings, of course. But if there are no any backups left from R77.30, I cannot really see it happenning.
If it is a gateway, then the easiest way is a clean install again and import of Gaia settings, which could be saved from R80.10.