update_subclass_type.rb

class A < ActiveRecord::Base
   def update_attributes_changing_type(attrs) 
    object = if attrs.keys.include?('type') || attrs.keys.include?(:type)
      self.becomes((attrs['type'] || attrs[:type]).constantize)
    else
      self
    end
    object.class.instance_variable_set('@finder_needs_type_condition', :false)
    object.update_attributes(attrs)
    object.class.instance_variable_set('@finder_needs_type_condition', nil)
    object
  end
end