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Since change from the top seems unlikely. Civil society should establish programs to reach out and provide alternative career options for current police officers who are willing to leave their jobs.
For example, they may help them transform into lawyers. NGOs may fund masters programs in different Law schools, provide them with extra tuition help, enroll them in human rights courses and help them establish small combined legal offices through grants.
Priority can be set to younger officers, ones with a publicly clean record. NGOs may also provide legal aid (or insurance) for old cases of police brutality or torture that might be raised against them after they leave the force.
Such NGO, and that's quite challenging, can transform into a self-sustainable association chaired by former officers.
The idea is to encourage officers who are not happy with the current status quo and are contemplating leaving but have limited or no options.