Make it so ranks cannot buy ranks or upgrades below them. For example the only thing an Elite rank can buy, is the Elite to Supreme upgrade, and nothing else. I know this is possible as you cannot buy Supreme to God as a VIP rank. This will prevent downgrades.
The prevention of downgrades is already in place, goto the shop, and try and purchase the rank below you, "You are not allowed to buy this" it says. Downgrading is impossible, buy craft has already stopped the problem.