Haven't promoted/demoted anyone though I have the priviledge to do so, but our fc have changed leaders at least 3 times and got all member ranks changed many many times by current leader. As far as I know all changes were made exactly as intended with members getting exact rank and privileges as our leader wanted them to be. Also fc is kind of based on trust to each other - if you don't know each other that well then:
1) do not keep more than 1 000 000 in fc chest.
2) give promotion/demotion rights only to members that you are 100% sure that they can be trusted and/or are not plain dumb. Try to keep the number of members with privilege to promote/demote as low as possible