>> You can't create SYSDBA role if there already are objects owned by SYSDBA
>> (or grants to SYSDBA). Try it.
>
> INSERT INTO RDB$ROLES ?

Right. I wanted to avoid to discuss all possible corruptions
users can do to their databases by direct updating system tables.
If somebody is so "clever" that prevents both SYSDBA and db owner
from login then he just gets what he deserves :-)

Ivan