Blowing efuses won't harm your device; in this context, they're like regular memory that can only ever be written to once. If too many fuses are blown, the software will assume that means a downgrade happened, and the software will refuse to boot.You will never run out of eFuses, then the system would never work in the end. That's the reason if you upgrade and downgrade, therefore zaps them. You would end up with a physically broken unit.
If there is someway to downgrade, it will be with an external device to hook and bypass the checks.