replace_method.rb

class Nyan
  def replace_method(a, b)
    (class << self; self end).module_exec do
      alias_method "#{a}_backup", a
      alias_method "#{b}_backup", b
      alias_method b, "#{a}_backup"
      alias_method a, "#{b}_backup"
    end
  end

  def with_replacing_method(a, b)
    replace_method(a, b)
    yield
    replace_method(b, a)
  end

  def nyan
    :nyan
  end

  def nyan_extended
    :nyan_extended
  end
end

nyan = Nyan.new

p nyan.nyan #=> :nyan

nyan.with_replacing_method(:nyan, :nyan_extended) do
  p nyan.nyan #=> :nyan_extented
end

p nyan.nyan #=> :nyan