Poacher turned gamekeeper
Definition 1 of 1
Pro Tip 1/3Role Reversal
Describes someone who switches sides, often having insider knowledge which makes them effective in their new role. 
The former spy is a poacher turned gamekeeper now, using his skills to catch other spies.
Pro Tip 2/3Figurative Use
The phrase is used metaphorically and is not about literal poaching or gamekeeping. 
She argued against the tobacco industry but as a poacher turned gamekeeper, now advocates for smoking rights.
Pro Tip 3/3Unexpected Transition
The switch in roles is often surprising or ironic, given the person’s past. 
The notorious car thief became a poacher turned gamekeeper, running a car security business.