secessionist (n.)

• apostate
• backslider
• breakaway
• collaborationist
• collaborator
• convert
• defector
• deserter
• mugwump
• proselyte
• quisling
• recidivist
• recreant
• renegade
• reversionist
• separatist
• strikebreaker
• tergiversator
• traitor
• turnabout
• turncoat