old_contents: PERMISSION_CHOICES = (
      ('read', 'Read'),
      ('write', 'Read and Write'),
  )
  
new_contents: READ_PERMISSION = 'read'
  WRITE_PERMISSION = 'write'
  
  PERMISSION_CHOICES = (
      (READ_PERMISSION, 'Read'),
      (WRITE_PERMISSION, 'Read and Write'),
  )
  
subject: Add constants for permission read and write values