I have just tried changing one of my admin to a War Chief and I am given a "invalid role " notification. I then demoted the same admin to a soldier and that works fine. Then retried promoting this member again to War Chief but got the same Invalid Role" message.
The basic soldier rank is out as it has no special permissions and functionality wise it's the same as member. All other ranks should be released within the next 12 hours. This is why you get the error but can demote from admin.