A person whose job or role has completely changed, from one extreme to another. Primarily heard in UK. The notorious computer hacker who once stole money from unsuspecting Internet users now helps the government catch other hackers—it's a strange case of poacher turned gamekeeper.
Prov. The best person to guard something is someone who knows all about how to steal it, so he or she can anticipate what thieves might do. We should hire the computer hacker to design computer security systems. An old poacher makes the best gamekeeper.