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I can't think of one directly, but an ex-thief who is entrusted a key is called 'a poacher turned gamekeeper' (for example, a computer hacker who becomes an adviser on network security, or a government tax officer who becomes an accountant specializing in tax).
Doh! Of course - that's it. I've also seen 'Putting King Herod in charge of Mothercare', but in the context (a rather intellectual newspaper article, or maybe a chat show) it was just a journalistic flourish; it may catch on, but I wouldn't call it an idiom yet.