require 'rubygems'
require 'mechanize'

a = WWW::Mechanize.new { |agent|
  agent.user_agent_alias = 'Mac Safari'
}

a.get('http://www.usj.com.my/gallery/add_comment.php?set_albumName=album49&id=I&gallery_popup=true') do |page|
  form = page.forms.first

  node = form.form_node
  thisinp = nil

  node.xpath(%q"//input[not(@type) or @type!='hidden']").each do |input|
    next unless input[:name]
    if thisinp.nil?
      thisinp = input
    else
      puts 'replacing'
      input.replace(thisinp)
    end

    puts input.class
    puts input
  end

  puts thisinp
end

Nokogiri::XML::Element
<input name="commenter_name" value="" size="30">
replacing
Nokogiri::XML::Element
<input name="save" type="submit" value="Post comment">
<input name="commenter_name" value="" size="30">