P2 (Front-end) Categories for all posts

Using the excellent tutorial by Nobble (http://wordpress.org/support/topic/351569) I was able to add a drop down category field to my P2 post box.

What I now want to do is add similar functionality to the existing posts listed in the blog.

I.e. I want to give each one of my posts a drop down box (easy to do following the instructions above) and then on changing the value of the drop down, the post will be updated and the new category value assigned. Is this last bit that Im having trouble figuring out.

Is there a way to have an ajax onchange function that will update the blog post when I select a category from the drop down?