title: Update paths of already-created Paperclip attachments
body: <p>I used to have this buggy Paperclip config:</p>
  
  <pre><code>class Photo &lt; ActiveRecord::Base
  
    has_attached_file :image, :storage =&gt; :s3,
                      :styles =&gt; { :medium =&gt; "600x600&gt;", :small =&gt; "320x320&gt;", :thumb =&gt; "100x100#" },
                      :s3_credentials =&gt; "#{RAILS_ROOT}/config/s3.yml",
                      :path =&gt; "/:style/:filename"
  end
  </code></pre>
  
  <p>This is buggy because two images cannot have the same size and filename. To fix this, I changed the config to:</p>
  
  <pre><code>class Photo &lt; ActiveRecord::Base
  
    has_attached_file :image, :storage =&gt; :s3,
                      :styles =&gt; { :medium =&gt; "600x600&gt;", :small =&gt; "320x320&gt;", :thumb =&gt; "100x100#" },
                      :s3_credentials =&gt; "#{RAILS_ROOT}/config/s3.yml",
                      :path =&gt; "/:style/:id_:filename"
  end
  </code></pre>
  
  <p>Unfortunately this breaks all URLs to attachments I've already created. How can I update those file paths or otherwise get the URLs to work?</p>