What would you use to flash the firmware? Why is downgrading not allowed?
You use Odin to flash it. Or KIES. Samsung does not allow downgrades, since you might downgrade to a firmware that is rootable or buggy.
Also for cross flashing Vodacom and generic firmware, they don't care. As long as the warranty flag by Knox is intact they are fine.
And most of the time Vodacom will deny your repair due to water damage in anycase.
This shouldn't be news to anyone really, the Samsung threads on here are filled with the Knox and flashing issues.