original source by @sasamijp

namechanger.rb

 # -*- encoding: utf-8 -*-

require 'twitter'

class UpdateString < String
  def delete_account_name(sasamljp)
    self.sub("(#{sasamljp})",'')
  end
  def delete_at_mark
    self.gsub(/(@|＠)/,'')
  end
  def escape_text
    CGI.unescapeHTML(self)
  end
end

 class NameChanger
   def initialize(status)
     @status=status
     @sasamljp='@sasamijp'
     @text=UpdateString.new(status.text)
     @user=status.user
   end
   public
    def update
      if is_change_order?
        @text=@text.delete_account_name(@sasamljp).delete_at_mark.escape_text
        send_update(@text)
      end
    end
   private
   def send_update(update_text)
     Twitter.update_profile(name:update_text)
     Twitter.update(get_notice_text(update_text),{'in_reply_to_status_id'=>status.id.to_s})
   end
   def get_notice_text(text)
     "@#{@user.screen_name} #{text}に改名しました"
   end
   def is_change_order?
     include_reply? and not is_unofficial_retweet?
   end
   def include_reply?
     @text.include?("(#{@sasamljp})")
   end
   def is_unofficial_retweet?
     @text.include?('RT')
   end
 end

namechanger_spec.rb

# -*- encoding: utf-8 -*-
require 'rspec'
require './namechanger'

describe 'UpdateString Test' do
  it 'リプライが消せる' do
    update_string=UpdateString.new('ササムルジェイピー(@sasamijp)')
    update_string.delete_account_name('@sasamijp').should eq 'ササムルジェイピー'
  end
  it 'アットマークが消える' do
    update_string=UpdateString.new('@sasamijp')
    update_string.delete_at_mark.should eq 'sasamijp'
  end
  it 'エスケープできる' do
    update_string=UpdateString.new('<sasamijp>')
    update_string.escape_text.should eq '<sasamijp>'
  end
  it 'メソッドチェーンできる' do
    update_string=UpdateString.new('&lt;&lt;&lt;&lt;@@@@@@ササムルジェイピー(@sasamijp)')
    update_string.delete_account_name('@sasamijp').delete_at_mark.escape_text.should eq '<<<<ササムルジェイピー'
  end
end

twitter_update.rb

# -*- encoding: utf-8 -*-
require 'rubygems'
require 'tweetstream'
require 'twitter'
require './namechanger'
require './key.rb'

Twitter.configure do |config|
  config.consumer_key        = Const::CONSUMER_KEY
  config.consumer_secret     = Const::CONSUMER_SECRET
  config.oauth_token            = Const::ACCESS_TOKEN
  config.oauth_token_secret  = Const::ACCESS_TOKEN_SECRET
end
TweetStream.configure do |config|
  config.consumer_key        = Const::CONSUMER_KEY
  config.consumer_secret     = Const::CONSUMER_SECRET
  config.oauth_token            = Const::ACCESS_TOKEN
  config.oauth_token_secret  = Const::ACCESS_TOKEN_SECRET
  config.auth_method            = :oauth
end
client = TweetStream::Client.new
client.userstream do |status|
  name_changer=NameChanger.new(status)
  name_changer.update
end