metaskillz.rb

module Test
  class One     
    def instance
      puts "instance"
    end
    
    def self.class_meth
      puts "class"
    end   
  end
end

module Override
  module Test
    
    def self.included(base)
      base.send :extend, ClassMethods
      base.send :include, InstanceMethods
      
      base.class_eval do
        alias_method :old_instance, :instance
        alias_method :instance, :new_instance
        
        class << self
          alias_method :old_class_meth, :class_meth
          alias_method :class_meth, :new_class_meth
        end     
      end
      
    end
    
    module InstanceMethods
      def new_instance
        puts "overridden instance"
      end   
    end
    
    module ClassMethods
      def new_class_meth
        puts "overridden class_meth"
      end
    end
  end   
end

Test::One.send(:include, Override::Test)
Test::One.new.instance # => "overridden instance"
Test::One.class_meth # => "overridden class_meth"